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COLOR 
AND ITS APPLICATIONS 



BY 

M. LUCKIESH 

DIRECTOR OF APPLIED SCIENCE, NELA RESEARCH LABORATORIES 
NATIONAL LAMP WORKS OF GENERAL ELECTRIC CO. 

AUTHOR OF " LIGHT AND SHADE AND THEIR APPLICATIONS," " THE LIGHTING 

ART," " THE LANGUAGE OF COLOR," "ARTIFICIAL LIGHT, ITS INFLUENCE 

UPON CIVILIZATION," "LIGHTING THE HOME," ETC. 



150 Illustrations — 4 Color Plates — 34 Tables 




SECOND EDITION 
ENLARGED 



NEW YORK 
VAN NOSTRAND COMPANY 

EIGHT WARREN STREET 
I92I 



COPYRIGHT I915, I921, BY 
D. VAN NOSTRA ND COMPANY 



THE PLIMPTON, PRESS 
NORWOOD -MASS-U-S 'A 



PREFACE 

The aim of this book is to present a condensed 
treatment of the science of color. An attempt has 
been made to cover as many phases of the subject 
as possible within the confines of a small volume. 
During several years of experimental work in the 
science of color I have been brought into contact with 
many persons interested in its applications, and the 
desire has been frequently expressed for a book that 
treated the science of color as far as possible from 
the viewpoint of those interested in the many applica- 
tions of color. These applications are constantly in- 
creasing in scope and interest. With this viewpoint 
in mind I have attempted to treat the subject, exercis- 
ing my judgment in drawing freely from the work of 
other investigators in order to make the volume as 
comprehensive as possible. I do not feel that the 
work comprises a complete treatment, for there are 
many interesting phases of color science that have 
been barely touched upon, and some that have been 
purposely omitted, because of the danger of straying 
too far afield. It is believed, however, that this 
treatise will be helpful to those interested in any of 
the arts involving the science of color. I have 
referred to my own investigations quite freely, but 
trust that this will not be attributed to a lack of per- 
spective. Naturally much of the text involves my 
own conclusions, but I have aimed to include only 
those that are supported by experimental data, be- 
cause only in so far as they are thus supported does 

ill 



iv PREFACE 

the work become authoritative. Many unsolved prob- 
lems have arisen throughout the text, which em- 
phasizes the need for more workers in the field. No 
attempt has been made to present a complete bib- 
liography of even the recent work in this branch of 
science; but references have been given freely, which, 
if followed, will provide a substantial beginning to the 
almost endless chain of material available. 

It is a pleasant duty to record my acknowledg- 
ments to the management of the National Lamp 
Works of the General Electric Company, whose broad- 
minded spirit in establishing the Nela Research 
Laboratory has made this work possible, and to the 
director of the laboratory and members of the staff, 
who always have given freely of their time and 
counsel. 

SECOND EDITION 

Some changes have been made in the original text 
and an extensive chapter has been added. This con- 
sists of useful data and methods for their use. 

M. LUCKIESH 
September, 1920 



CONTENTS 
CHAPTER I 

Page 
LIGHT 1 

Wave Theory. Electro-magnetic Theory. Radiation and Light Sensa- 
tion. Temperature and Radiation. Spectra of Ulimiinants. 

CHAPTER II 

THE PRODUCTION OF COLOR 23 

Refraction. Diffraction. Interference. Polarization. Reflectioiij, Ab-_- 
sorption, and Transmission. Color of Daylight. Color Sensations 
Produced by Colorless Stimuli. Fluorescence and Phosphorescence. 
Useful Filters. 

CHAPTER III 

COLOR-MIXTURE 54 

Subtractive Method. Additive Method. Juxtapositional Method. 
Simple Apparatus for Mixing Colors. 

CHAPTER IV 

COLOR TERMINOLOGY. 69 

Hue, Saturation, and Brightness. Tri-color Method. Color Notation. 

CHAPTER V 

THE ANALYSIS OF COLOR 86 

.The Spectroscope. The Spectrophotometer. The Monochromatic 
Colorimeter. The Tri-chromatic Colorimeter. Other Methods. 
Templates. Reflectometer. Methods of Altering Brightness Non- 
selectively. 

CHAPTER VI 

COLOR AND VISION 116 

The Eye. B^^htness Sensibility. Hue Sensibility. Saturation Sensi- 
bility. Visui Acuity in Lights of Different Colors. Growth and Decay 
of Color Sens tions. Signaling. Other Uses for Colored Glasses. 



Vi CONTENTS 

CHAPTER VII 

THE EFFECT OF ENVIRONMENT ON COLORS 163 

niiimination. After-images. Simultaneous Contrast. Irradiation. 

CHAPTER VIII 

THEORIES OF COLOR VISION 181 

Young-Helmholtz. * Duplicity.' Hering. Ladd-Franklin. Edrigde- 



Green. 



CHAPTER IX 



COLOR PHOTOMETRY 191 

Methods of Color Photometry. Other Means of Eliminating Color Dif- 
ferences. Direct Comparison and Flicker Methods. Luminosity 
Curve of the Eye. 

CHAPTER X 

COLOR PHOTOGRAPHY 213 

Lippmann Process. Wood Diffraction Process. Color Filter Processes. 



CHAPTER XI 

COLOR IN LIGHTING 224 

Artificial Daylight. Units for Imitating Daylight. Effect of Colored Sur- 
roundings. Color in Interiors. Color Preference. A Demonstration 
Booth. 



CHAPTER XII 

COLOR EFFECTS FOR THE STAGE AND DISPLAYS 272 

Stage. Displays. 

CHAPTER XIII 

COLOR PHENOMENA IN PAINTING 282 

Visual Phenomena. Lighting. Pigments. 

CHAPTER XIV 

COLOR MATCHING , 302 

The niuminant. The Examination of Colors. 



CONTENTS vii 

CHAPTER XV 

THE ART OF MOBILE COLOR 312 

Color Music. Its Relation to Sound Music. 

CHAPTER XVI 

COLORED MEDIA 327 

Available Coloring Materials. Dyeing. Gelatine Films. Solvents. 
Lacquers. Celluloid. Phosphorescent Materials. Miscellaneous 
Notes. 



CHAPTER XVn 

CERTAIN PHYSICAL ASPECTS AND DATA " . . . . 344 

Three Types of Colored Media. Pigments. Optical Properties of Pig- 
ments. Applications of Spectral Analyses of Pigments. Reflection- 
factors of Pigments. Spectral Analyses of Dye-solutions. Applications 
of Spectral Analyses of Dyes. Laws Pertaining to Colored Solutions. 
Dichromatism. Graphical Method for Using Spectral Data. Spectral 
Analyses of Glasses. Red, Yellow, Green, Blue, and Purple Glasses. 
Use of Spectral Analyses of Glasses. Influence of Temperature on 
Transmission of Colored Glasses. Ultraviolet Transmission of Media. 
Compounds Sensitive to Temperature. Transmission of Light by Fog 
and Water. Color Temperature of Illuminants. 

INDEX 407 



COLORED PLATES 

Prismatic Spectrum Frontispiece 

Diffraction Grating Spectrum " 

Subtractive method of mixing colors Facing page 54 

Additive method of mixing colors " " 54 

Showing the effect of environment on the appearance of colors " " 163 
Illustrating the effect of the spectral quality of the illuminant 

Daylight, below; ordinary artificial light, above . . " " 282 



LIST OF ILLUSTRATIONS 

Figure Page 

1. Radiation curve of an incandescent solid 8 

2. Showing the relation between radiant energy and hght sensation ... 10 

3. Showing the effect of temperature on the radiation from an incandes- 

cent solid (black-body) 12 

4. Representative spectra 17 

5. Distribution of energy in the visible spectra of various illuminants . . 20 

6. Newton's experiment 23 

7. Effect of the character of the slit of a spectrograph on the grating spec- 

trimi of the mercury arc 24 

8. Dispersion curves of various optical media 26 

9. Young's double-slit experiment illustrating the principle of the diffrac- 

tion grating 26 

10. Diagrammatic illustration of polarized hght 31 

11. The Nicol prism for obtaining plane-polarized light 33 

12. Analyses of ordinary colors 36 

13. Showing the variation in the spectral character of sunlight due to at- 

mospheric absorption 38 

14. Benham disk for producing subjective colors by means of black and 

white stimuli 39 

16. Diagrammatic illustration of the action of the rhodamine fluorescent 

reflector 44 

16. Spectrophotographic analysis of the action of the rhodamine fluores- 

cent reflector 46 

17. Screens for producing lights of the same hue but diflfering in spectral 

character 48 

18. Ultra-violet spectra 60 

19. Ultra-violet spectra 61 

20. The subtractive method of mixing colors (colored plate) 

21. The additive method of mixing colors (colored plate) 

22. The color-wheel for showing complementary hues . 69 

23. Maxwell disks 62 

24. An erratic color-mixing disk 64 

25. A simple color-mixer 64 

26. A simple color-mixer for transparent or opaque media 66 

27. Lambert's color-mixer 66 

28. A shadow demonstration of the additive and subtractive methods of 

color-mixture 66 

29. Illustrating a disk for approximating a prismatic spectrum 68 

30. Disk * a,' for varying only the satiu-ation of a color. — Disk * b,' for vary- 

ing only the brightness of a color 71 

31. The Maxwell color-triangle 73 

32. Spectral complementaries 76 

33. A color pyramid 76 

34. The double pyramid (after Titchener) 76 

36. A demonstration color-triangle 76 



LIST OF ILLUSTRATIONS 



36. The A. H.Munsell color tree 81 

37. Prang's color and brightness scales 82 

38. Ruxton's color mixture chart for prmting inks 82 

39. A direct-vision prism spectroscope 86 

40. A simple grating spectroscope 86 

41. The spectrophotometer. 88 

42. The Nutting pocket spectrophotometer 88 

43. A small portable spectrophotometer for quantitative analysis .... 89 

44. The variable sectored disk (after Hyde) 90 

45. Scheme for reducing the amoimt of spectrophotometric work in ex- 

amining transparent colored media 91 

46. Abney's spectrophotometric attachment for a spectrometer 93 

47. Ives' spectrophotometric attachment for a spectrometer 93 

48. Nutting's spectrophotometric attachment for a spectrometer .... 94 

49. The Nutting monochromatic colorimeter 95 

60. Analysis of two component color-mixtures 99 

61. A simple method of converting a spectrometer into a combined mono- 

chromatic colorimeter, direct comparison photometer, flicker pho- 
tometer, and spectrophotometer 100 

62. Illustrating the principle of the Maxwell ' color box ' 101 

63. The F. E. Ives colorimeter 103 

64. Koenig's sensation curves 104 

65. Tri-color colorimeter measurements 104 

66. Arrangement for using color filters before a photometer eyepiece . . 106 

67. Arons colorimeter 108 

68. Abney's template for carmine 110 

69. Adaptation of Abney's scheme for the spectroscopic synthesis of color 111 

60. The Nutting refiectometer 113 

61. A vertical section of the human eye 116 

62. Showing the effect of chromatic aberration in the eye 118 

63. A simple achromatic lens 119 

64. Limits of the visual field for colored and colorless lights 120 

65. Brightness sensibility data. (See Table X) 121 

66. Hue sensibility. (Steindler's Eye) 125 

67. Hue sensibility, limen, and color scale 126 

68. Apparatus for determining visual acuity in monochromatic lights . . 133 

69. Visual acuity in monochromatic lights of equal brightness 135 

70. Visual acuity in the mercury spectrum, the lines being reduced to equal 

brightness 136 

71. The growth and decay curves for white light sensation. (Broca and 

Sulzer) 138 

72. The growth and decay curves of color sensations 139 

73. Showing the maxima attained by flickering Ughts at various frequencies 140 

74. Showing the maxima of sensations produced by flickering red light 

on a steady green field (R), and vice versa (G) 141 

75. Showing the relation between brightness and critical frequency for 

colored stimuli 145 

76. Effect of contour of flicker on critical or vanishing-flicker frequency . 147 

77. Effect of yellow-green glasses on vision imder a bright sky 155 

78. Ultra-violet transmission curves of various glasses 158 

79. Effect of the intensity of illumination on the appearance of a pigment 165 

80. Illustrating why a purple appears differently under two different 

illuminants 167 

81. Effect of brightness on the duration of the after-image 171 



LIST OF ILLUSTRATIONS xi 

82. Showing the effect of simultaneous contrast. The V*s are of equal 

brightness 174 

83. Showing induction. Each band, though uniform in brightness, appears 

brighter at the right-hand edge 176 

84. An arrangement for showing the reduction in the contrast effect by 

separating the two colored objects 176 

85. An arrangement for showing the effect of simultaneous contrast and 

after-images 176 

86. Illustrating irradiation 179 

87. The evolution of the Ladd-Franklm gray molecule 187 

88. The results of four methods of photometry. (Ives) 196 

89. Spectral sensibilities of seleniimi and photo-electric cells compared 

with the spectral sensibility of the eye 200 

90. Spectral sensibility of a panchromatic photographic plate 202 

91. An accurate color filter for the panchromatic plate considered in Fig. 90. 203 

92. Results by flicker and direct comparison photometers, illustrating dif- 

ferences including the Purkinje effect and a reversed effect . . 206 

93. VisibiUty data. (See Table XVI) 209 

94. Illustrating the standing waves produced in the Lippmann process. . 216 

95. Illustrating the Wood diffraction process 216 

96-98. Illustrating three processes of color photography 219 

99. Illustrating the limitations of certain processes of color photography . 220 

100. Ideal transmission screens for producing artificial daylight 230 

101. Showing the loss of light when using the ideal artificial-dayUght screens 

with the tungsten lamp operating at 7.9 lumens per watt .... 231 

102. Showing the loss of light when using the ideal artificial-daylight screens 

with the timgsten lamp operating at 22 lumens per watt .... 232 

103. Showing the spectral analyses of two subjective white lights com- 

pared with the spectral analysis of noon sunlight 236 

104. Showing the additive method of producing artificial daylight .... 236 

105. Showing the relative amounts of light of the character of A and B 

(Fig. 104) necessary to produce artificial daylight by addition . . 237 

106. Illustrating the effect of multiple selective reflection of light from a 

green fabric 248 

107. Showing the relative proportions of red, green and blue components in 

the reflected Ught from a green fabric after various successive 
reflections 249 

108. Screen for altering timgsten light to the same spectral character as 

carbon incandescent electric light ; c, d, e show the transmission 

curves of amber glasses of different densities 264 

109. Comparison of ideal screen a, Fig. 108, with amber glass 256 

110. Showing the preference or rank of a ntmiber of fairly saturated colors . 261 

111. Wiring diagram of an experimental and demonstration booth . . . ." 267 

112. Showing dimensions and locations of lamps in the demonstration 

booth 268 

113. Illustrating the effect of colored light upon the appearance of six 

colored papers 273 

114. Illustrating the changing of scenery by the use of colored lights . . . 276 

115. Illustrating the disappearing effects produced on a specially painted 

scene by varying the color of the illuminant 276 

116. Illustrating a flashing sign produced by properly relating the hue and 

brightness of the pigments with the color of the illuminant. . . 279 

117. Showing the reflection coefficents of fairly saturated colors for day- 

light and tungsten incandescent electric light. (See Table XV) . . 286 



xii LIST OF ILLUSTRATIONS 



118. Showing the effect of the illuminant upon the appearance of a colored 

frieze 288 

119. Showing the effect of the spectral character of the illuminant upon the 

values of a painting 290 

120. Effect of distribution of light on the expression of a painting 293 

121. Illustrating the optics of picture lighting 294 

122. Spectral analyses of pigments 298 

123. Spectral analyses of pigments. 298 

124. Dlustrating the effect of the amount of the green components in blue 

and yellow pigments on the amoimt of ' black ' in the mixtures . 299 
126. Diagrammatic illustration of the results of mixing blue and green pig- 
ments containing various amounts of green 300 

126. The * Luce ' part for the ' Clavier k lumieres ' in Scriabine's * Pro- 

metheus * 315 

127. Illustrating an instrument for studying the emotive or affective value 

of colors and color phrases ; Rimington's color code also shown 323 

128. A color-mixture instrument for studying the emotive and affective 

value of colors and color phrases 324 

129. Showing the relative positions of the colored lamps in the apparatus 

diagrammatically shown in Fig. 128 325 

130. Michrophotographs of white cotton and silk fabrics against a black 

background 348 

131. Spectral reflection-factors of pigments 352 

132. Spectral reflection-factors of pigments 353 

133. Spectral luminosities of pigments 360 

134. Spectral luminosities of pigments 361 

135. A study of a pigment (light chrome yellow) 362 

136. Reflection-factors of pigments 368 

137. Relative reflection-factors of pigments 369 

138. Influence of the illuminant on the appearance of a pigment 370 

139. Relation between spectral transmission-factor and depth or concen- 

tration of a solution of methylengriin 381 

140. Relation between spectral luminosity and depth or concentration of a 

solution of rosazeine 382 

141. Complete relation between thickness, wave-length, and transmission- 

factor for a gold ruby glass 383 

142. Spectral transmission-factors of selenium glasses 387 

143. Spectral transmission-factors of copper, sulphur, chromiimi, and 

uranium glasses 388 

144. Spectral transmission-factors of gold glasses and combinations with 

cobalt ' . 389 

145. Spectral transmission-factors of carbon glasses and combinations 

with cobalt glasses 390 

146. Spectral transmission-factors of cobalt glasses 391 

147. Spectral transmission-factors of iron and of manganese glasses . . . 392 

148. Relations between spectral transmission-factor and thickness of a 

gold glass (23Au) 393 

149. Relations between spectral luminosity and thickness of a gold glass 

(23Au) 394 

160. Test of the relation between spectral transmission-factor and thick- 
ness of a blue-green glass 395 



COLOR 

AND ITS APPLICATIONS 



CHAPTER I 
LIGHT 

1. The word Light has acquired two meanings; 
one pertains to sensation and is therefore physiological 
and psychological in character, while the other refers 
to the external cause of the sensation and is there- 
fore physical in nature. As both meanings are used 
in the study of color, they will be distinguished wher- 
ever necessary; for example, light rays when imping- 
ing upon the retina of the eye produce the sensation 
of light. In order to understand the phenomena of 
color, a fair knowledge of the physical nature of 
light must first be acquired. Unfortunately the field 
to be covered in this book is too extensive to permit 
of a detailed treatment of this interesting subject; 
only those phenomena will be discussed that are 
essential to an understanding of the subsequent 
chapters. Those wishing to pursue this line of study 
further can readily do so by consulting the many 
excellent treatises on the subject. 

2. Wave Theory, — The passage of a beam of 
light from a source (flame, sun, etc.) to a receiver 
(the earth, the eye, etc.) involves a transfer of energy, 
and the question arises as to how this transfer takes 
place. All around us in Nature, energy is contin- 
ually being transferred from one place to another 
and whenever such a transfer does occur something 
is moving. On the ocean, for example, energy is 



COLOR AND ITS APPLICATIONS 



transferred by water due to its wave motion. Moun- 
tain streams are carrying energy, which fortunately 
can be made to do useful work by means of a water 
motor, but here the energy is transferred by the 
onward flow of the water. In air the same two meth- 
ods of transferring energy are found; currents in 
the case of winds and waves in the case of sounds. 
Solids also can be made to transmit energy in these 
two ways; by currents as in the sand blast and by 
waves as in the case of sound and other elastic dis- 
turbances. Since currents and waves are such com- 
mon methods of transmitting energy, it is quite 
natural that they should be called upon to explain the 
transfer in the case of light. Light travels in straight 
lines, casts (comparatively) sharp shadows, is reflected 
from a smooth surface as a regular succession of 
rubber balls would be if thrown against the same 
surface, and in many other ways acts much like a 
current of particles would act if projected from the 
source of light at a high velocity. 

There is one phenomenon, however, that cannot 
be explained by the assumption of a current of par- 
ticles; under certain conditions two rays of light of 
equal intensity can be sent to the same spot in such 
a manner that the spot will be dark and not twice as 
bright as it would be if either ray were present alone. 
This fact is explained by assuming that light energy 
is transmitted in the form of wave motion for it is 
seen that if two equal waves are made to pass in 
the same direction through any medium, but in such 
a manner that the crests of one wave coincide with 
the troughs of the other, the two waves will annul 
each other everywhere, there will be no resultant 
wave, no transfer of energy, and hence no light at 
the spot in question. To this phenomenon was given 



LIGHT 



the name * interference,' but the term has been 
extended to include all the phenomena that may take 
place when two or more waves travel in the same me- 
dium at the same time. The foregoing case and all 
others in which there is destruction of motion are 
now grouped under the term, destructive interference; 
in contradistinction to this, there is constructive 
interference wherever the motion due to all the 
waves is greater than that due to one. The simplest 
case of the latter type is that of two equal wave trains 
traveling in the same direction at the same time but 
in such a manner that the crests of one coincide with 
the crests of the other. The two waves reinforce 
each other and the resultant wave has twice the am- 
plitude of the original waves. Another very important 
special case of interference is that to which the term 
* standing wave' or * stationary wave' has been applied. 
This occurs whenever two equal wave trains are 
passing in opposite directions through the same me- 
dium at the same time. The most common way of 
obtaining equal waves traveling in opposite directions 
is by means of reflection at a surface perpendicular 
to the direction in which one train is traveling. Stand- 
ing waves can be readily demonstrated by fastening 
one end of a long rope (preferably so that it hangs 
vertically) and by shaking the other end, timing the 
motion of the hand so that it is in unison with the 
reflected waves. It will be seen that some points 
of the rope remain at rest and others swing through 
a large amplitude. The points at rest are called 
nodes and the part of the string between the nodes 
is a segment. By varying the speed of the hand or 
the period of vibration the string can be made to 
vibrate in one, two, three, or more segments. 

A brief consideration of wave motion in general 



COLOR AND ITS APPLICATIONS 



and a few definitions may not be out of place here. 
In the first place, it is evident that in any wave 
motion, the parts of the medium do not travel as far 
as the wave. They remain each in its own region, 
each causing adjacent parts to move and in so doing 
gives up to the adjacent part some of its energy. The 
motion of the particles differ in various kinds of 
waves. " In waves in deep water each drop moves in 
a vertical plane in a circular orbit. In waves in shal- 
low water the orbit is an elongated ellipse. In 
media transmitting sound waves the motion of each 
particle is to and fro in a straight line in the direction 
in which the wave is traveling. For all waves the 
wave-length is the distance between any two suc- 
cessive particles that are moving through the same 
points in their orbits at the same instant. The ampli- 
tude is half a particle's path length (the diameter 
of the orbit). The period is the time taken for the 
wave to travel one wave-length. The frequency is 
the reciprocal of the period or the number of waves 
that pass a given point in a unit of time. If two 
waves are *in step' so that a crest of one occurs at 
the same time and place as a crest of the other, 
the two waves are said to be in phase. If a wave is 
confined to a surface such as that due to a pebble 
dropped in a quiet pond of water, the waves will be 
circular; any circle is a wave front , and the direction 
in which the wave is traveling at any point is that of 
the radius drawn to the point and is therefore per- 
pendicular to the wave front. In the case of light 
under ideal conditions, the wave will spread out in 
all directions from a point, so that the wave front 
will be spherical. The direction of propagation will 
again be perpendicular to the wave front along the 
radii of the sphere. 



LIGHT 5 

Light energy or radiant energy passes through a 
vacuum. The phenomenon of interference has been 
explained by wave motion. Hence it is assumed that 
there is something in the vacuum that can move. 
This something is called the ether and is further 
assumed to penetrate all matter so that light waves 
always are ether waves; the properties of the waves 
may change as the matter imbedded in the ether is 
changed, but it is the ether and not the matter 
imbedded in the ether that is responsible for the 
propagation of the light waves. Some scientists, 
are not reconciled to this view but fortunately in this 
treatise we need not enter into the discussion. 

The adoption of the wave theory necessitates 
new and somewhat elaborate explanations for such 
simple phenomena as the rectilinear propagation of 
light; these have been made by the aid of Huy- 
ghen's principle, which states that each point on a 
wave front may be regarded as a new source of dis- 
turbance, sending out spherical waves, and that at 
any instant the new wave front will be the envel- 
ope of all of these secondary wavelets. By the aid 
of this principle it is at once evident that a light 
wave in going through a wide slit will pass on in 
such a manner that the sides of the slit cast a rather 
sharp shadow, whereas in going through a very narrow 
slit, comparable in width with a wave-length of light, 
it will pass on and spread out in all directions thus 
* turning a corner.' This phenomenon has been 
termed diffraction. 

It is helpful to visualize light waves by means 
of the water wave analogy as has been done in the 
foregoing, but it is well to guard against being misled 
by following the analogy too closely. For example 
water waves diminish in amplitude on account of 



6 COLOR AND ITS APPLICATIONS 

dissipation of energy through molecular friction. In 
free space the amplitude of light waves (which is a 
measure of their intensity) has not been observed 
to decrease; in other words, there is no friction in 
the transmitting medium. 

3. Electro-magnetic Theory, — At first it does not 
appear that there is any relation between light and 
electricity, but such a relation was predicted by Max- 
well in about 1870. This theory assumes light rays 
to be identical with the electro-magnetic disturbances 
which are radiated from a body in which electrical 
oscillations are taking place. Some years later Hertz 
actually produced these waves and by this discovery 
the electro-magnetic theory expounded by Maxwell 
was supplied with the necessary physical foundation. 
In enunciating this theory it is customary to state 
that the oscillating electrons in the atoms which 
constitute a body send forth through space pulses 
of electro-magnetic energy. The electron at present 
is supposed to be an atom of electricity. These 
electric waves emanating from a radiating body 
whether it be the sun or a red-hot poker have many 
properties depending upon their wave-length. Al- 
though all travel at the same velocity, about 3 x lo ^^ 
centimeters (186000 miles) per second, in free space, 
they differ somewhat in velocity in the ordinary trans- 
parent media. In glass, for instance, violet rays travel 
less rapidly than the red rays. All these rays represent 
energy and therefore regardless of wave-length have 
the property of producing heat when absorbed. Some- 
times the energy is not wholly converted into heat 
but enters into chemical reactions or is converted 
into electricity or radiant energy of other wave-lengths 
than those of the absorbed rays. Some of the rays, 
especially those of shorter wave-length than the visible 



LIGHT 7 

rays, are very active chemically, affecting a photo- 
graphic emulsion, destroying bacteria and animal, 
tissues such as the outer membrane of the eye 
and causing sunburn; they are also largely respon- 
sible for exciting phosphorescence and fluorescence. 
Other rays have varying effects on organisms and 
play a more or less important part in the growth of 
plants. Rays within a certain range of wave-lengths, 
separately and in groups, produce the sensation of 
light and color. In other words the retina of the 
eye can be likened to a receiving station in wireless 
telegraphy which is * tuned' to respond to electro- 
magnetic rays of a certain (limited) range of wave- 
lengths. 

4. Radiation and Light Sensation. — There are 
many ways of decomposing radiation into its various 
component rays. The rainbow is the result of one 
of Nature's means of dispersing the radiation from 
the sun into rays of various wave-lengths. The eye 
sees in the rainbow many colors, the most conspicu- 
ous being violet, blue, green, yellow, orange and red. 
These are seen to be of different brightnesses. If 
the retina were sensitive to rays of an infinite range 
of wave-lengths the rainbow would appear much wider 
than it does. That is, colors whose appearance 
can not be imagined, would be seen in the now in- 
visible regions beyond the violet and red, because 
energy corresponding to those wave-lengths is present 
in sunlight. 

The distribution of the energy among the different 
wave-lengths radiated by a hot solid is shown in 
Fig. 1. This curve is technically known as a radia- 
tion curve and shows that energy of a great range 
of wave-lengths is present. Such a continuous spec- 
trum is characteristic of the radiation from solid 



8 



COLOR AND ITS APPLICATIONS 



bodies. On the basis of the electro-magnetic theory 
of light, it may seem strange that rays of all wave- 
lengths are produced by the vibrating electrons. 
This may be pictured sufficiently well for the present 
purpose by assuming that in a solid body there is 
considerable damping of the vibrations and that other 
influences are present which result in the emission 




WAVE LENGTH 



Fig. 1. — Radiation curve of an incandescent solid. 

of no characteristic single ray or series of rays but, 
instead, of rays of all wave-lengths. The height of 
the curve at any point above the axis of abscissae or 
base line represents the relative amount of energy of 
that particular wave-length present in the total radia- 
tion. It will be noted that the amounts of energy of 
various wave-lengths are by no means of the same 
value. This characteristic of illuminants is of great 
importance in a study of colors as will become evident 
later. The region to which the eye is * tuned,' the 
visible spectrum, lies between V (violet) and R (red) 
which is exaggerated in relative extent for the sake 
of clearness. 

The relation between radiation and light sensa- 
tion is not simple. The ability of the various rays 
to produce light sensation is shown roughly by the 
dotted curve in Fig. 1. The maximum light sensa- 
tion is produced by rays in the middle of the visible 
spectrum, namely by those giving rise to a yellow- 



LIGHT 9 

green sensation. Beyond the limits of the visible 
spectrum, V and i?, it is obvious that an infinite 
amount of electro-magnetic energy causes no sensa- 
tion of light. The total range of wave-lengths might 
be called the energy spectrum of this particular radi- 
ator. It is obvious that the greater the percentage 
of the total radiant energy confined to the visible 
spectrum, the greater is the * luminous efiiciency ' of the 
radiating body. In the production of light the total 
range of wave-lengths is of interest, but in the con- 
sideration of Color, interest is very largely confined 
to the visible spectrum. 

As the temperature of an incandescent body is 
increased, the energy of the shorter wave-lengths 
increases more rapidly than the energy of the longer 
wave-lengths. Considering the visible spectrum, the 
violet and the adjacent rays increase in intensity 
more rapidly with increase of temperature than the 
red and its adjacent rays thus causing the light 
emitted by an incandescent filament, for instance, to 
become bluer as its temperature is increased. Here 
it is well to note that when the various visible rays 
are permitted to impinge separately upon the retina 
each produces its own color sensation but when all 
the visible rays simultaneously stimulate the retina, 
as in the ordinary viewing of colorless objects, an 
integral sensation is produced. In the case of most 
common illuminants the integral sensation is an 
unsaturated yellow, that is, a yellowish white, while 
the combined sensations aroused by average day- 
light produce the integral sensation of white light. 

In the discussion of Fig. 1 it has been seen that 
energies in the various wave-lengths differ in light- 
producing effects. To this must be added the fact 
that the light-producing effect varies with the inten- 



10 



COLOR AND ITS APPLICATIONS 



sity. In Fig. 2, A represents the light sensation pro- 
duced in the author's eye by equal amounts of energy 
of various wave-lengths as measured with a direct 
comparison or equality-of-brightness photometer. The 
photometric field was a circle whose diameter sub- 




0.40 0.44 



(M Q52 Q56 050 

M, WAVE LENGTH 



0.64 



0.6& 



Fig. 2. — Showing the relation between radiant energy and light 
sensation. 

tended an angle of about four degrees at the eye, and 
whose brightness was equivalent to that of a white 
surface illuminated to an intensity of 20 meter candles ; 
(a meter candle is the illumination received on a 
surface everywhere one meter distant from a source 
of one candle, the surface being perpendicular to 
the straight line from it to the light source). On 
decreasing the intensity and therefore the brightness 
of the photometer field to about one two-hundredth 
of its original value or to an equivalent of 0.1 meter 
candle on the foregoing basis, the relation between 
light and radiation become as shown in curve B. 
It will be noted that the maximum of the luminosity 
curve (as it is called) has shifted toward the shorter 



LIGHT 11 

wave-lengths. In other words at the low illumina- 
tion the light-producing effect of visible rays of the 
shorter wave-lengths has not decreased as much as 
that of the longer wave-lengths. To illustrate by 
a simple experiment, suppose a red and a blue sur- 
face appear of equal brightness at a high illumina- 
tion. If the intensity of illumination is reduced to 
a very low value, the blue surface will appear much 
brighter than the red one. To further complicate 
matters it is found that even normal eyes differ some- 
what in spectral sensibility for experiment shows that 
the luminosity curves for various normal eyes do not 
exactly coincide (see #56). Curve C is plotted from 
Koenig's ^ data obtained at a very low illumination, 
practically at the threshold of vision. This phenome- 
non of shifting spectral sensibility which was discov- 
ered by Purkinje, and which bears his name, will be 
discussed in later chapters, and the quantitative rela- 
tion between radiation and light sensation will be fur- 
ther treated in the chapter on color photometry. 

5. Temperature and Radiation. — As already 
stated the effect of raising the temperature of a 
heated solid body is to increase the luminous effi- 
ciency and also the relative amount of energy in 
the rays of shorter wave-lengths. These effects are 
shown diagrammatically for a solid body in Fig. 3. 
The numbers on the curves indicate the absolute 
black-body temperatures. The wave-length is given in 
terms of ten-thousandths of a centimeter, this unit 
being usually expressed by the Greek letter, /x. The 
rays to which the eye is sensitive lie between V and /?, 
respectively about 0.4/x and 0.7^. The eye in reality 
is sensitive somewhat beyond these wave-lengths but 
for practical purposes the amount of light sensa- 
tion produced by rays beyond these limits is usually 



12 



COLOR AND ITS APPLICATIONS 









/ 


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negligible. It is seen that as the temperature rises 
the maximum of the radiation curve shifts toward 
the shorter wave-lengths. The maximum of the radi- 
ation curve of sunlight lies in the visible region. 
This has brought forth the suggestion that the eye 

80 

70 
>. 60 
S 50 

ZL 
U 

ui 40 

°= 20 

10 


I 

Jl, WAVE LENGTH 

Fig. 3. — Showing the efifect of temperature on the radiation 
from an incandescent solid (black-body). 

in its process of evolution has become most sensitive 
to the rays of such wave-length as are a maximum 
in the radiation from the sun. As the maximum of 
the radiation curve shifts toward the shorter wave- 
lengths, it is seen that a relatively greater proportion 
of the total energy is found in the visible region 
between V and R which accounts for the increase in 
luminous efficiency. One of the tendencies in light 
production is toward the development of materials 
and methods which will enable the light source to be 
operated at higher temperatures in order to appease 
the ever-present demand for higher luminous effi- 
ciencies. It is evident that the ideal light source 
emitting a continuous spectrum would be one that 
radiated no energy beyond V and R. The area 
under each curve is proportional to the total amount 
of energy emitted by the radiator at a certain tem- 



LIGHT 13 



perature and the ratio of the area under that part 
of any curve included between V and R is propor- 
tional to the energy that can effect the eye. The 
ratio of the latter area to the former (for the same 
curve) is called the * radiant efficiency' of the radiator 
as a light source. To make the idea of radiant effi- 
ciency of practical value it must be combined with the 
relations between luminous sensation and radiation. 

6. Spectra of Illuminants. — The spectral distribu- 
tion of energy in the radiation from different illumi- 
nants is of great importance in the consideration of 
color owing to the fact that the appearance of the 
colored objects depends upon the spectral character 
of the illuminant under which they are viewed. The 
variation in the spectral character of illuminants is 
due to the temperature and composition of the radi- 
ating body and also to the state in which it exists 
when radiating luminous energy. 

A simple means of producing light is that of 
heating a solid conductor by passing an electric 
current through it. At first it will emit invisible 
radiant energy known as infra-red rays. As the 
temperature is raised it will finally become luminous, 
at first appearing a dull red. This is evident from an 
inspection of Fig. 3. If these light rays be studied 
by means of a spectroscope which disperses the 
radiation into its component rays, it will be found 
that deep red rays are the only visible rays present 
in appreciable amounts. As the temperature is in- 
creased the appearance of the body passes from red 
to orange, then to yellow and so on. If the body 
were sufficiently refractory to withstand higher tem- 
peratures and remain in solid form, at a certain tem- 
perature it would appear white and with increasing 
temperature would assume a bluish white appearance. 



14 COLOR AND ITS APPLICATIONS 

The latter temperatures have never been reached in 
the production of artificial light. Notwithstanding 
the fact that all solids produce a continuous spectrum 
and obey the general laws mentioned, it does not 
follow that they all emit the same amounts of light 
per unit area at equal temperatures. Kirchhoff has 
shown by the theory of exchanges that the emissive 
and absorptive powers of all bodies at the same tem- 
perature for rays of a particular wave-length are pro- 
portional to each other when the radiation is a pure 
temperature effect. For a particular kind of radiator 
called a black body or a full radiator, the relation 
between emission E, the wave-length, X, and the 
absolute temperature J, has been deduced theoretic- 
ally. The black body is defined as a body that 
will absorb all radiation incident upon it and reflect 
none. When it radiates it emits in each wave-length 
more energy than any other body at the same tem- 
perature. The nearest approach to such an ideal 
radiator is a hollow space enclosed by emitting walls 
of uniform temperature provided with a small open- 
ing through which the radiation can escape. The 
laws deduced theoretically by various investigators 
do not agree entirely. The one that best fits exper- 
imental data is Planck's law given in 

Ex= CA-'(e^-l)-' (1) 

Another law known as the Wien-Paschen law found 
to hold for the short-wave region of the visible spec- 
trum is shown in equation (2). 

Ex= Ci\-' e ^T (2) 

In the foregoing equations Ci and C2 are constants. 
The values for these differ somewhat as determined 
by various investigators. 



LIGHT 15 



A simple relation between the wave-length of the 
maximum of the radiation curve and the absolute 
temperature is derived from (2) and is expressed in 
equation (3). 

KT = constant (3) 

Another interesting relation known as the Stefan- 
Boltzmann law connects the total radiation, E, from 
a unit area of the radiator with the absolute temper- 
ature, T, and is expressed in equation (4). 

E = CT' (4) 

These laws are of chief importance in the theory of 
radiation but are given here as a matter of reference. 
A gaseous body is found to emit only certain 
definite rays and the spectrum is said to be a line 
spectrum. Sometimes the various lines (which are 
in reality the images of the slit of the spectroscope, 
(8) are found to be crowded together in such a 
manner as to give to the spectrum a fluted appear- 
ance. Such a spectrum is called a banded or fluted 
spectrum. A further striking fact is the constancy 
of the appearance of the spectra emitted by elements 
in gaseous form. For instance the spectrum of 
sodium is always recognized by the position of the 
emitted rays in the spectrum — that is, by their 
wave-length. The visible spectrum of sodium con- 
sists of a double line (0.5890m and 0.5896^) and 
whenever this double line is found in a spectrum it 
is certain that sodium is present in the radiating 
substance. This constancy of the spectra of the 
elements forms the basis of spectrum analysis by 
means of which traces of elements far too small to 
be weighed by the most sensitive balance are readily 
detected. By means of the spectroscope helium was 
discovered on the sun before it was distinctly isolated 



16 COLOR AND ITS APPLICATIONS 

by scientists on earth. The vacuum tube, the arc, 
the electric spark, and the flame are used in studying 
the spectra of elements and compounds. 

Sometimes both a line and a continuous spectrum 
are emitted by an illuminant. Such a case is found 
in the ordinary carbon electric arc. The crater of 
the arc being an incandescent solid, emits all visible 
rays while the incandescent gas of the arc between 
the electrodes emits a line spectrum the appearance 
of which depends upon the surrounding medium and 
the composition of the carbons. In Fig. 4 are shown 
several representative spectra photographed by means 
of a grating spectrograph on a Cramer spectrum plate 
which is sensitive in varying degrees to all the vis- 
ible rays. This particular brand of photographic 
plate is relatively less sensitive to blue-green rays 
so that on viewing the spectrograms the energy in 
this region appears to be less prominent than it 
really is. It is seen that the two gases, mercury 
and helium, emit line spectra. The arcs emit both 
continuous and line spectra, the latter as indicated 
above being emitted by the vapor in the arc itself. 
The relative prominence of the line spectra depends 
upon the relative intensities of the radiation from 
the arc as compared to that from the solid electrodes. 
For instance the line spectrum is much more prom- 
inent in the yellow flame arc than in the ordinary 
carbon arc and as is well known the arc vapor con- 
tributes a much greater proportion of the light in the 
former than in the latter illuminant. The line spec- 
trum of a carbon arc is subject to momentary changes 
both in character and intensity due to impurities 
and also to irregularities in the amounts of the chem- 
icals with which the carbons are impregnated. The 
injection of various chemicals into the arc as sug- 



^//w- I \/j^}f:,iQ Spectrum 




a. tiercurcj Arc 



b. Helium 



c. Iron Arc 



d. Yellow Flame Arc 



e. Carbon Arc 



f. Carbon Arc 



^H g. Carbon Arc 



h. Tungsten Incandescent Lamp 



I. SKijIight 



j, Skij light 



UltroA VBOYOR 
Violet I Visible Spectrum 



Fig. 4. — Representative spectra. 
17 



18 COLOR AND ITS APPLICATIONS 

gested by the heating of metallic salts in a Bunsen 
flame, affords a means of varying the color or spectral 
character of the light from the carbon arc lamps. 
Spectra / and g were obtained from the same carbon 
arc within a period of a few seconds. The tungsten 
filament is seen to emit a continuous spectrum, /i, 
the dark band being due to the low sensibility of the 
photographic plate to blue-green rays. Two spectro- 
grams of light from the sky are shown in i and j 
in an effort to bring out the dark lines which cross 
the spectrum. 

The solar spectrum is of special interest. As 
already indicated a photograph of the spectrum of 
sunlight made with a narrow slit, shows practically 
a continuous band crossed by many fine dark lines 
(see Plate I). These lines were discovered by 
Wollaston in 1802 but were later studied with better 
instruments by Fraunhofer in 1814 who found several 
hundreds of them. These dark lines in the position 
of various colors show the absence of the corre- 
sponding images of the slit of the instrument and 
therefore the absence of these rays in sunlight. 
Their absence is attributed to absorption by vapor 
chiefly in the solar atmosphere. Luminous gases or 
vapors, as has already been indicated, emit only a 
limited number of rays, their spectra being dis- 
continuous in appearance. These vapors when lumi- 
nous are usually opaque to the particular rays which 
they emit and therefore the light from the sun is 
robbed of some of the rays in passing through its 
atmosphere. The Fraunhofer lines are often used as 
reference points in examining spectra, although 
electric discharges through gases in vacuum tubes and 
the heating of salts in a gas flame furnish convenient 
means of identification or reference spectra in the 



LIGHT 



19 



experimental laboratory, 
are given in Table I. 



The chief Fraunhofer lines 



TABLE I 
Principal Fraunhofer Lines 



Line 


Wave-length 


Color 


Source 


A 


0.7594M 


Red 


Oxygen in atmosphere 


a 


.7185 


<( 


Water vapor in atmosphere 


B 


.6867 


(( 


Oxygen in atmosphere 


C 


.6563 


(( 


Hydrogen, sun 


Di 


.5896 


YeUow 


Sodium, " 


D2 


.5890 


(( 


<( n 


E 


.5270 


Green 


Calciiun, " 


bi 


.5184 


{( 


Magnesium, ** 


b2 


.5173 


({ 


(( (( 


b4 


.5168 


11 


(( ({ 


F 


.4861 


Blue 


Hydrogen, " 


G 


.4308 


Violet 


Calcium, " 


H 


.3969 


(( 


(( (i 


K 


.3934 


(t 


It it 



Other convenient lines obtained by heating salts 
in a Bunsen flame are — potassium red, 0.7699 and 
0.7665 m; lithium red, 0.6708; sodium yellow, 0.5896 
and 0.5890; thallium green, 0.5351; magnesium 
green, 0.5184 ; strontium blue, 0.4607. The mer- 
cury arc gives a double yellow line, 0.5790 and 0.5764; 
a bright green line, 0.5461; a faint blue-green line, 
0.4916; an intense blue line, 0.4358; and deep violet 
lines, 0.4078 and 0.4047. Hydrogen at a fairly low 
pressure in a glass tube through which an electric 
discharge is passed yields a red line, 0.6563, blue- 
green, 0.4861, and blue 0.4341. The helium spec- 
trum obtained from a tube such as the foregoing is 
rich in useful lines throughout the spectrum, yielding 
red lines 0.7282, 0.7065, 0.6678; yellow 0.5876; 
green lines 0.5048, 0.5016, 0.4922; blue lines 0.4713, 



20 



COLOR AND ITS APPLICATIONS 



0.4472, 0.4388; and violet lines 0.4026, 0.3888. Iron, 
copper, zinc, and cadmium when used as the ter- 
minals of an electric spark yield many useful lines, 
many of which are in the ultra-violet. The iron arc 
and the quartz mercury arc are particularly useful 
for exploring the ultra-violet region. Three useful 
cadmium lines are red 0.6438, green 0.5086, and blue 
0.4800. 

280 
260 
240 
220 
200 
180 
^ 160 



140 



120 



< 

a; 100 

60 
60 
40 



20 





























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04& 0.52 0.56 060 
^^.WAVEL LENGTH 



0.64 



0.66 



Fig. 6. — Distribution of energy in the visible spectra of 
various illuminants. 

In Fig. 5 and Table II are shown the spectral dis- 
tributions of energy in the visible region for various 
illuminants. These data have been obtained in the 
Nela Research Laboratory chiefly by Hyde,^ Ives,^ 
Cady and the author.^ As is evident at a glance these 



LIGHT 



21 



TABLE n 
Relative Distribution of Energy in the Visible Spectra of Common Uluminants 





A 


B 


C 


D 


E 


F 


G 


H 


I 


1 




! 


1 


j! 


g 

1 


it.! 


flii 

Sl<i 


1 
d 




0.41m 


72. 
79. 


i77. 
186. 


1.9 
3.6 


4. 
7. 


6.6 
9.6 




16.6 
22.5 






.43 


21.8 




.46 


84.3 


187. 


6. 


12. 


16. 


16.7 


30. 


29. 


17.6 


.47 


91. 


180. 


10.6 


18. 


21.9 


23.5 


38. 


37. 


26.4 


.49 


92.6 


162. 


16.3 


25.6 


30.3 


32.7 


47. 


45.6 


38.3 


.61 


96. 


146. 


26.6 


34.6 


40. 


42.6 


56.5 


55. 


51. 


.53 


98. 


132. 


37.6 


47. 


62. 


54.9 


67. 


66.5 


64. 


.66 


99. 


120. 


63.2 


62. 


66.6 


68.6 


78. 


76. 


78. 


.67 


100. 


108. 


74.6 


79. 


82. 


83.4 


88. 


88. 


90. 


.69 


100. 


100. 


100. 


100. 


100. 


100. 


100. 


100. 


100. 


.61 


100. 


93. 


130. 


123. 


118. 


117. 


111. 


113.5 


107. 


.63 


98.6 


87. 


168. 


148. 


139. 


136. 


121.5 


127. 


111. 


.66 


97.1 


82. 


210. 


176. 


160. 


167. 


131. 


142. 


114. 


.67 


96.6 


77. 


260. 


204. 


182. 


179. 


140. 


156. 


119. 


\69 


93.6 


72.6 


320. 


234. 


205. 


202. 


147.5 


170. 


120. 



curves are plotted in such a manner that the relative 
energy of wave-length 0.59 m equals 100. This method 
of plotting gives the relative distribution of energy 
for approximately the same amounts of total light 
sensation. Reference to the spectral distribution 
of energy in the radiation from the two tungsten 
lamps, operating at 7.9 and 22 lumens per watt 
respectively, shows the effect of increasing temper- 
ature upon the relative amounts of rays of shorter 
wave-length emitted. See Table XXIII. 



22 COLOR AND ITS APPLICATIONS 

REFERENCES CHAPTER I 

1. Ges. Abhandlungen zur PhysioL Optik. Leipzig, 1903, p. 144 et 

seq. 

2. Jour. Franklin Inst., 191 o, p. 439. 

3. Trans. I.E.S., 1910, p. 189. 

4. Elec. World, Sept. 19, 1914; Trans. I.E.S., 1914, p. 839, 

OTHER REFERENCES 

Theory of Optics. Preston. 

Physical Optics. R. W. Wood. 

Outline of Applied Optics. P. G. Nutting. 

Lectures on Illuminating Engineering. Johns Hopkins University. 



CHAPTER II 
THE PRODUCTION OF COLOR 

7. Colors and color phenomena are encountered 
in nearly every human activity. In fact they are so 
ever-present that they have become common-place 
to the average man excepting in those instances 
where they enter actively into his work. That there 
is an explanation for every case of color production 
is inevitable, however, there are some color phe- 
nomena as yet unexplained satisfactorily. Color is 
produced in many ways. It is intimately associated 
with light as has already been seen. When color 
is produced it is usually the result either of a sub- 
traction of some of the visible rays from the total 
radiation emitted by light source or of the dispersion 
of the visible radiation into its component parts. 

8. Refraction. — Sir Isaac Newton in 1666 dis- 
covered that sunlight consisted of many rays each of 
which when permitted to impinge separately upon the 
retina produced the sensation of a distinct color. 
He accomplished 
this by prismatic 
dispersion and 
proved that no 
further change 

resulted from Fig. 6. — Newton's experiment. 

subsequent dis- 
persion. Newton's experiment, which is shown dia- 
grammatically in Fig. 6, was performed approximately 
in the following manner. A prism was set up in a 

23 




24 



COLOR AND ITS APPLICATIONS 



darkened room and sunlight admitted through a small 
hole in the window shade. This provided him with a 
parallel beam of light. After the beam traversed the 
prism he found that there was no longer an image of 
the sun, but a colored band similar to the rainbow, 
made up in reality of an infinite number of colored 
images of the slit overlapping each other. On passing 
a narrow portion of this colored band through a hole in 
another screen and permitting this to traverse another 
prism, he found no further change in color, thus prov- 
ing that monochromatic light can not be further de- 
composed. The principles involved in this experiment 
are used today in a great deal of spectroscopic work. 

Let us examine some of the characteristics of the 
spectrum somewhat more in detail. It will be found 
that an increase in the width of the slit produces an 
increase in the brightness of the spectrum but owing 
to the facts that the spectrum consists of overlapping 
images of the slit and that each image of a broad 

slit is wider than an image of 
a narrow slit, it is evident 
that there will be more over- 
lapping of the images pro- 
duced with a wide slit. The 
smaller the amount of over- 
lapping the purer the spectrum 
will be. The difference in the 
appearance of the spectrum 
of a mercury arc due to a 
change from a narrow rectan- 
gular slit to a wide circular opening is shown in Fig. 7; 
both spectra were photographed on the same grating 
spectrograph. The material of which a prism is made 
also has a marked effect on the appearance of the spec- 
trum. When a ray of light passes through a prism it is 



V B 



6 Y 





Fig. 7. — Effect of the character 
of the slit of a spectrograph on 
the grating spectrum of the 
mercury arc. 



THE PRODUCTION OF COLOR 



25 



of course bent out of its original direction, the amount 
of bending or the angle of deviation depending upon 
the angle of the prism and the index of refraction for 
the particular wave-length of which the ray consists. 
If for a given prism the angle of deviation remained the 
same while the wave-length of the light was changed, 
then the index of refraction would also remain con- 
stant, and if wave-lengths were plotted against either 

1.72 

1.70 
1.68 
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.yU. WAVE LENOTH 

Fig. 8. — Dispersion curves of various optical media. 

angles of deviation or indices of refraction the disper- 
sion curve would be a straight line. If such a substance 
actually existed, it would be possible to determine 
wave-lengths in its spectrum by the use of an ordinary 
graduated scale. However, such is not the case and 
further, the dispersion curves of various prisms differ 
considerably. The dispersion curves of quartz and 
three kinds of glass are presented in Fig. 8. It is 
evident from an inspection of these curves that the 
different wave-lengths are more closely crowded to- 
gether in the red or long wave-length end of the 



26 



COLOR AND ITS APPLICATIONS 



spectrum than in the other. This is shown in Plate 
I (Frontispiece) where a prismatic spectrum is com- 
pared with a normal (grating) spectrum in which equal 
distances represent equal wave-length intervals. 

For the study of the visible rays transparent glass 
prisms are satisfactory but for the study of the invis- 
ible rays other media must be used because glass is 
not sufficiently transparent for very short or very 
long waves. Hence investigations of the ultra-violet 
region are made with optical systems of quartz and 
those of the infra-red with fluorite or rock salt. Ex- 
tremely long waves such as are used in wireless 
telegraphy are studied by means of huge prisms of 
pitch or paraffin. 

9. Diffraction. — Another common device employed 
in optical instruments for decomposing visible radia- 
tion into its components is the 
so-called diffraction grating. A 
grating is simple in appearance 
but is difficult to make for it 
consists of a great many paral- 
lel lines (sometimes as many 
as 40,000 and more per inch) 
scratched usually upon glass or 
speculum metal. When a grat- 
ing is placed in the path of a 
beam of light a spectrum re- 
sults, which, unlike a prismatic 
spectrum, has a constant dis- 
persion and is therefore called 
Fig. 9.-Young's doubie-sUt ex- ^ normal spectrum. 

periment illustrating the princi- To explain the actiou of the 

pie of the diffraction grating. ^jff ^.^ction grating we shall Con- 
sider only two of the very large number of slits of which 
the grating consists. In Fig. 9 has been sketched an 




THE PRODUCTION OF COLOR 27 

instantaneous view of a section perpendicular to the 
two slits. A source of light, S, has sent out spherical 
monochromatic waves, successive fronts being repre- 
sented by the full lines. The dotted lines drawn mid- 
way between the full lines must then represent parts of 
the disturbance exactly in opposite phase with the wave 
fronts. If the diagram be considered for a moment as 
representing a surface of water into which a stone has 
been dropped at S, then the full lines might represent 
the crests of the waves and the dotted lines the 
troughs. One wave front is represented as contain- 
ing the two slits. Si and S2. It was stated in # 2 that 
according to Huyghen's principle any point on a wave 
front may be regarded as a center of disturbance. 
The wave front originally proceeding from S strikes 
the screen and cannot pass through excepting for 
the two small parts of this wave front that strikes 
the openings Si and S2. These set up the two sys- 
tems of spherical waves represented on the right 
hand side of the screen. It has also been stated (# 2) 
that, whenever two or more systems of waves travel 
in the same medium at the same time, interference 
results. Wherever two wave fronts intersect (two 
crests in the water analogy) there will be an especially 
large disturbance; wherever two of the dotted lines 
intersect there will be an especially large disturbance 
(an especially deep trough in the water analogy). 
Where a wave front meets a part of the disturbance 
in opposite phase, that is wherever a full line inter- 
sects a dotted line, there will be destructive inter- 
ference and consequently no motion. The heavy 
full lines have been drawn through those points 
where there is maximum motion and the heavy 
dotted lines through those points where there is no 
motion and therefore no light. Now suppose these 



28 COLOR AND ITS APPLICATIONS 

waves be permitted to impinge upon a screen repre- 
sented by the right hand edge of the diagram. There 
will evidently be light on the screen wherever a 
heavy dotted line terminates. 

Imagine that the source S has been sending out 
blue light and is now exchanged for one that emits 
red light. A very similar diagram will result but on 
account of the longer wave-length, the ends of the 
heavy lines will no longer intersect the screen at the 
points Oi and O2; there will still be light at Oo as is 
evident from the symmetry of the diagram, but the 
new points corresponding to Oi and O2 will lie some- 
what farther from Oo than in the case of blue light. 

Finally, suppose the source S to emit white light 
(light of all visible wave-lengths). Then there will 
be a white spot at Oo and spectra at Oi and O2, and 
the blue edge of these spectra will be nearest Oo. 
In other words, in a grating spectrum the long wave- 
lengths are deviated more than the short ones, 
which is opposite to the condition that exists in prism 
spectra. Further, it will be seen that the distances 
in the spectra, Oi, O2, etc., are proportional to the 
wave-length; in other words, that a grating spectrum 
is one of constant dispersion. (See Plate I.) 

The spectra, Oi, are said to be of the first ordeVj 
O2 of the second order, and so on. It is also evident 
that if more heavy lines had been drawn in the dia- 
gram the positions of the third and higher order 
spectra would also have been indicated. It will be 
further noted that the higher the order the greater 
is the length of the spectrum, and therefore the 
greater is the dispersion. 

The colors of some crystals such as the fiery opal, 
and of insects and feathers are often accounted for 
by the phenomenon of diffraction. On looking at 



THE PRODUCTION OF COLOR 29 

an arc lamp through a screen door or the meshes of 
an umbrella top, diffraction spectra are seen. Like- 
wise on viewing a light source over a thin edge of 
an opaque object held close to the eye, a colored 
fringe is seen. These bands are due to the inter- 
ference of light waves. Since the wave-length of 
red light is greater than that of violet light, the red 
light penetrates further into the shadow than the 
violet light. 

The first gratings were made by Fraunhofer in 
1821, and consisted either of fine wire or of fine 
rulings on a smoked glass. At present gratings are 
ruled by means of a diamond on glass and on the 
bright reflecting surface of speculum metal. Row- 
land at Johns Hopkins University contributed much 
toward the production of satisfactory gratings. On 
his machine as high as 110,000 lines per inch can be 
ruled, but usually the number does not exceed 15,000 
or 20,000. Cheap copies of gratings are obtained by 
flowing a film of celluloid dissolved in amyl acetate 
over a grating, afterward stripping this off and mount- 
ing it between plate glasses. It is evident that in 
the case of rulings upon an opaque substance such 
as speculum metal the spectra must be produced by 
reflection and not by transmission as discussed above. 
The individual rulings always act as absorbing bodies 
while the unruled portions either reflect or transmit 
this light. 

10. Interference. — Other color phenomena be- 
sides that of the diffraction-grating spectrum arise 
from the process of interference. Thin films of trans- 
parent substances such as oil on water, soap bubbles, 
thin sheets of mica, and iridescent crystals, owe 
their color usually to the interference of light waves. 
If a slightly convex glass surface be placed upon a 



30 COLOR AND ITS APPLICATIONS 

plane piece of glass, colored bands known as Newton^s 
rings will be seen. These are due to interference 
between the light waves reflected from the upper 
and lower surfaces respectively of the thin film of 
air of varying thickness between the two pieces of 
glass. When white light is incident normally at the 
point of contact the colored bands are circular and 
concentric with the point of contact. These bands 
are violet on the inside and red on the outside but 
at a short distance from the center they begin to 
overlap and gradually disappear. If monochromatic 
light is used the bands appear of the color of the light 
and many bands can be seen. In the case of some 
crystals such as chlorate of potash and fiery opals, 
the colors are found to be very pure. The colors of 
insects and feathers are often accounted for by the 
phenomena of interference. The process of color 
photography devised by Lippmann (# 57) is based upon 
the principle of interference of light waves. 

11. Polarization. — Imagine for the sake of sim- 
plicity a beam of sunlight emerging from an aperture 
in a window shade. All the different waves that 
make up this beam are traveling in the same gen- 
eral direction. As mentioned previously, light waves 
are transverse; that is, the particles of the medium 
that transmit the waves move to and fro in a straight 
line perpendicular to the direction in which the beam 
is traveling. In the simplest case all of the particles 
that transmit a wave move so as always to be in one 
plane. For example in a. Fig. 10, the particles Ay By 
C, Dy etc., move to and fro along perpendicular paths 
but the wave travels horizontally. If such a wave 
could be seen from one end it would appear simply as 
a short straight vertical line as indicated in b. Sup- 
pose that the beam of sunlight emerging through 



THE PRODUCTION OF COLOR 



31 



the window shade could be examined minutely end 
on. We would then see something similar to c for the 
different waves vibrate in all possible different planes. 
Suppose further that by some means the beam could 
be transformed so that when seen end on, it would 




Fig. 10. — Diagrammatic illustration of polarized light. 



appear like d. Such a beam consisting of waves 
in parallel planes is said to be a plane-polarized 
beam, and curiously, the nomenclature adopted states 
that such a beam in which the waves all move in 
vertical planes, is polarized in the horizontal plane. 
It is understood that these graphical diagrams are 
used for the sake of presenting the subject pictorially 
and with no claim that they represent actual condi- 
tions. Nevertheless if the actual conditions were 
such the results of experiments would readily be 
explained by the reasoning used here. One simple 
way of * polarizing' a beam of light is by permitting 
it to fall upon a plate of glass so that the angle of 
incidence is 57 degrees as shown in e. The unpolar- 
ized beam AB will be separated into refracted (BC) 



32 COLOR AND ITS APPLICATIONS 

and the reflected {BD) beams, and the former will be 
found to consist mainly of waves vibrating in the 
plane of the paper, as indicated by the cross lines 
representing the path of the particles, while the 
reflected beam will be found to consist of waves 
vibrating perpendicular to the plane of the paper as 
indicated by the dots representing the paths seen 
end on. To show that the beam BD really has 
different properties than 5 C it is only necessary to 
attempt to reflect each from another piece of glass. 
If the second piece of glass be placed at C in a 
position similar to the first, the beam BC will pass 
through but if it be rotated about C through an angle 
of 90 degrees keeping its angle with the beam con- 
stant it will be found that the beam BC will not be 
transmitted. The treatment for the beam BD is 
obvious. 

Another means of polarizing a beam of light is 
by permitting it to pass through certain crystals, 
such as tourmaline, Iceland spar and quartz. If 
this be done it will be found that the incident beam 
is divided into two beams polarized at right angles 
to each other, the two having different directions in 
the crystal, different velocities and different prop- 
erties in general. One of these beams will always 
be found to obey the ordinary laws of refraction, for 
example, that the incident ray, the normal to the 
surface, and the refracted ray all lie in one plane; 
the other beam will not obey the foregoing and other 
simple laws generally. The former is therefore 
called the ordinary ray and the latter the extraor- 
dinary ray. Nicol devised a very convenient method 
of separating the two beams when produced by a 
crystal of Iceland spar as illustrated in Fig. 11. If 
a rhomb of spar be cut into two parts along the plane 



THE PRODUCTION OF COLOR 



33 




indicated by the diagonal, PP, and if the parts be 
polished and cemented together with Canada balsam, 
the paths of the two beams will be as indicated. 
The Canada balsam has a refractive in- 
dex intermediate between that of the spar 
for the ordinary and extraordinary rays. 
Therefore the ordinary ray, O, being inci- 
dent upon the balsam layer at an angle 
greater than the critical angle, is totally 
reflected while the extraordinary ray is 
merely slightly refracted by the layer of 
balsam. 

That the beam emerging from the 
Nicol prism is really plane-polarized can 
be shown by a mirror, as before, or more 
simply, by a second Nicol prism. If the 
second prism be rotated through one com- 
plete revolution, two positions will be 
found for which the light transmitted by both prisms 
is practically of the same intensity as that transmitted 
by one, and two other positions will be found for which 
no light is transmitted. For intermediate positions the 
intensity of the light will be less than that transmit- 
ted by one Nicol. When the maximum amount of 
light is transmitted the Nicols are said to be * parallel' 
and when no light is transmitted they are * crossed.' 

Some substances, such as quartz (cut in a certain 
manner), sulphate of lime, sugar solution, and tur- 
pentine, behave in a peculiar manner when placed 
between crossed Nicols, for though no light passes 
the second prism before the introduction of the other 
substance, some light is transmitted when the sub- 
stance is inserted in the path. If monochromatic 
light is used the light can be extinguished by rotating 
either prism to the right or left by an amount depend- 



Fig. 11. — The 
Nicol prism 
for obtaining 
plane-polar- 
ized light. 



34 



COLOR AND ITS APPLICATIONS 



ing upon the substance, its state of concentration if 
in solution, and the thickness of the layer intro- 
duced. Such layers are said to rotate the plane of 
polarization. If various monochromatic lights of dif- 
ferent colors are used in succession it will be found 
that the prism must be rotated different amounts for 
the different colors in order to bring about a total 
extinction of light after the introduction of one of 
the substances. Hence if such a substance be ex- 
amined in white light, a certain position of the prism 
will extinguish the blue light but permitting the re- 
maining colored rays to pass in varying proportions; 
at another position yellow will be extinguished and 
the remaining rays permitted to pass in varying pro- 
portions, and so on. The transmitted light will of 
course have a different color in each case. As 
quartz is one of the chief substances having this 
property, the following table is given to show the 
magnitude of the rotation in degrees produced by 
two thicknesses of quartz for light of different wave- 
lengths. 



Thickness of 
quartz 


Red 


Orange 


Yellow 


Green 


Blue 


Violet 


1 mm. 


18° 
136 


22° 
161 


24° 
180 


30° 
218 


32° 
232 


42° 
316 



Another instance of the production of color by 
polarization is that of the examination between 
crossed Nicols of certain other crystals that trans- 
form plane-polarized light into so-called circularly or 
elliptically-polarized light. Many minerals occuring in 
Nature have either this property or that of rotating 
the plane of polarization, and an entire system has 
been developed for determining what substances are 



THE PRODUCTION OF COLOR 35 

present in a given rock by noting the color effects 
produced by a thin section of the rock in a micro- 
scope equipped with two Nicol prisms. 

12. Reflection^ Absorption^ Transmission. — Ordi- 
nary colors, which are encountered, are produced by 
selective reflection or transmission. Since most sub- 
stances do not have the property of reflecting the 
same proportions of all light rays received they are 
said to be selective in their reflection. A red fabric 
has the ability to reflect chiefly the red rays of the 
visible spectrum; therefore when white light falls 
upon it only the red rays are reflected while the 
remaining visible rays are absorbed. By this process 
of selective reflection the colors of pigments and most 
of the colors in Nature are produced. The same 
remarks apply to the production of color by selective 
transmission. Colors produced by these means are 
not as pure as the colors of the spectrum. Each 
of the latter consists practically of a single wave- 
length and are said to be monochromatic, while the 
colors ordinarily encountered in Nature are im- 
pure, consisting of rays of a considerable range of 
wave-lengths. 

In Fig. 12 are shown diagrammatically the spectral 
analyses of five common pigments. For each pig- 
ment the reflecting power (i.e. the per cent of 
energy reflected) was determined for all wave-lengths 
in the visible spectrum. The results plotted against 
the corresponding wave-lengths are represented by 
the full lines in the diagram. The dotted curves 
represent roughly the relative light values and are 
obtained from the full curves by multiplying the 
energy values represented by the ordinates by their 
relative abilities to produce light sensation (#4). 
A similar discussion applies to the same colors pro- 



36 



COLOR AND ITS APPLICATIONS 



duced by transmission. These analyses will be dis- 
cussed further in Chapter V. (See Figs. 122 and 123.) 
The character of the surface of a pigment and the 
density Of the coloring matter influence the appear- 
ance of a color. If, for instance, a red aniline dye 
solution be deposited upon a fabric by means of an 
air brush, the pigment under some conditions will 



Purple 



Blu£ 



Oreeh 



Yellow 



Red 




040 



050 0.60 

JJi, WAVELEMGTH 

Fig. 12. — Analyses of ordinary colors. 



be deposited in the form of a fluffy powder whose 
appearance is a deep velvety red. The purity of the 
red is largely due to multiple reflections, for the light 
is able to penetrate an appreciable distance into the 
medium (#64, 75), and at each reflection it becomes 
purer — that is, more nearly monochromatic. If the 
solution were applied by means of an ordinary brush, 
the deposit would not have been so porous and the 
appearance of the color would not have been such 
a deep red. Another instance of the effect of mul- 
tiple reflections on the color of light is found in a 
gold-lined goblet. All are familiar with the color 
of gold plating. This, however, becomes much more 



THE PRODUCTION OF COLOR 37 

reddish when inside a goblet, owing to the purifying 
effect of multiple reflections. 

The color of transparent media depends upon the 
depth of the coloring matter. For instance, a hollow 
glass wedge, when filled with an aqueous solution of 
ethyl or methyl violet, will appear bluish at the thin 
end and reddish at the thick end. Cyanine is an- 
other dye that exhibits dichroism, which is the name 
applied to the foregoing. The different appearances 
of silk and woolen fabric dyed in the same solution 
are due largely to the difference in the character of 
the surfaces. Many of these instances could be 
cited here, but the details will be treated in succeed- 
ing chapters. 

13. Color Due to Scattered Light, — That light 
is changed in color by being scattered by fine par- 
ticles is a fact observed daily. Tyndall, by precipi- 
tating clouds of vapor, observed that as the particles 
increased in size the bluish color of the clouds dis- 
appeared. Smoke from the end of a cigar appears 
bluish, yet the smoke exhaled appears whitish; this 
latter is perhaps caused by an increase in the size 
of the particles, due to the condensation of moisture. 
Rayleigh ^ has mathematically treated the problem of 
scattered light and experimented with a sulphur 
precipitate. He noted a polarizing effect which is of 
interest. J. J. Thomson ^ treated the scattering due 
to small metallic spheres theoretically. Garnett^ con- 
cluded that colored glasses owe their colors to the 
presence of microscopic spheres of the metal of the 
coloring agent. Colloidal solutions appear to act in 
the same manner. Mie ^ has extended Garnett's 
theory very considerably; however, the exact explana- 
tion of the color of glasses is still somewhat disputed. 

The color of daylight is of special interest because 



38 



COLOR AND ITS APPLICATIONS 



it is our most common illuminant. Light from the 
sky consists chiefly of scattered sunlight in daytime. 
If it were not for the finely divided particles of matter 
in our atmosphere, the sky would be quite as dark 
by day as by night. When the particles are of a 
size comparable with the wave-length of light rays, 
they produce considerable scattering of the latter. 
This scattering of light is selective, the rays of short 

100 




Q50 ceo 

M, WAVE LENGTH 

Fig. 13. — Showing the variation in the spectral character of sunlight due to 
atmospheric absorption. 

wave-length being scattered in greater amounts than 
those of longer wave-length; this accounts for the 
bluish color of the sky (Fig. 5). Sunlight is termed 
white light, but in many cases throughout this book 
the term * white' light is used to indicate the total 
light from an illuminant emitting all visible rays. 
Where necessary, the two meanings are differentiated. 
Direct sunlight, however, undergoes a change in 
color as the altitude of the sun changes, on account 
of the greater thickness of air, more or less laden 
with smoke, dust, vapor, and ice, through which 
the light must pass at the lower altitudes. This 
change in color caused by the combined actions of 



THE PRODUCTION OF COLOR 



39 



absorption and scattering, is such as to depress the 
blue, and hence the sun appears redder as it ap- 
proaches the horizon. All the brilliant colors of 
sunset are due to the foregoing phenomena. The 
effect of different masses of air upon the relative 
amounts of energy in each wave-length which reach 
a given point on the earth's surface has been deter- 
mined by Abney; his data are reproduced in Fig. 13. 
Of course the results obtained will depend upon the 
purity of the atmosphere. Over a smoky industrial 
city, the sun, when near the horizon, almost daily 
appears a fiery red. Often the absorption is so great 
that the sun disappears from view long before it 
actually sinks below the horizon. 

14. Color Sensations Produced by Colorless 
Stimuli. — Colors are often visible to a careful 
observer when not produced by any of the methods 
already mentioned. If a disk composed of black and 
white be rotated at the proper rate — moderately 
slow — colors appear upon the leading and lagging 
edges of the sectors. 

In other words when 
the black and white stimuli 
precede or follow each other 
at certain intervals, colors 
are produced instead of 
gray. Fechner, in 1838, was 
perhaps the first to describe 
these subjective colors, and 
his name was later applied 
to the phenomenon by 
Brticke. Many have stud- 
ied the problem and there 
is a general agreement as to 
the results obtained, though there is no such agreement 




Fig. 14. — Benham disk for producing^ 
subjective colors by means of black 
and white stimuli. 



40 COLOR AND ITS APPLICATIONS 

as to their explanation. Benham,^ in 1894, produced 
a disk different from those used by preceding investi- 
gators and made an attempt to solve the problem. 
One form of his disk, illustrated in Fig. 14, shows 
the colors in a striking manner when rotated. The 
phenomena can only be briefly discussed here. In 
general, when black is followed by white at a mod- 
erate speed, a sensation of red results, but if white 
be followed by black, a sensation of blue is experi- 
enced. By introducing various angular intervals, 
as is done in the Benham disk, sensations of inter- 
mediate colors are aroused. On rotating the disk 
in one direction the blue sensation is aroused in the 
inner ring and red in the outer; on reversing the 
disk, the colors aroused are also reversed in their 
order. The phenomena are interesting and have 
received a great deal of attention, though, as already 
stated, there is no general agreement as to their 
complete explanation. No doubt retinal inertia and 
the difference in the rates of growth and decay of 
the color sensations are important factors in the 
production of these so-called subjective colors. Often, 
when suddenly moving the eye over black and white 
surfaces in the field of vision, these colored effects 
are perceptible. A simple disk for showing the 
Fechner colors, though not as effective as the Benham 
disk, is one containing plain black and white sectors. 
On rotating such a disk at a certain speed it will 
appear of a greenish hue, but at a somewhat more 
rapid rate of rotation it appears a reddish hue. Helm- 
holtz used a white disk upon which was painted a 
black spiral. Rood ^ used an opaque disk with four 
open sectors, each of seven degrees. Through this 
rotating disk he viewed a clouded sky. With a rate 
of nine revolutions per second the sky appeared a 



THE PRODUCTION OF COLOR 41 

deep crimson hue, except for a small spot in the 
center of the visual field, which remained constantly 
yellow. This latter is probably due to retinal dif- 
ferences. The center of the retina, called the * yellow 
spot,' is known to exhibit selective absorption. At 
eleven and one-half revolutions per second the field 
appeared bluish-green. 

15. Fluorescence and Phosphorescence. — Usually 
the radiant energy absorbed by bodies is trans- 
formed into heat energy. There are many sub- 
stances, however, — some solid, some liquid, and some 
gaseous, — that have the property of absorbing radiant 
energy of certain wave-lengths and of emitting it 
again after transforming it into radiant energy of 
other wave-lengths (nearly always longer than those 
absorbed). 

The name fluorescence was derived from fiuor spar 
(calcium fluoride), which has long been known to 
possess the property of emitting light rays of a dif- 
ferent color than that of the rays with which it is 
illuminated. Strictly speaking, the term applies to 
the phenomenon only when it ceases immediately 
after the exciting light is extinguished; in this sense 
it is applicable to liquids and gases only. In solids 
the phenomenon usually continues for some time 
after the exciting light has been shut off. This 
prolonged emission, which in some cases lasts for 
hours, is termed phosphorescence. The phenomena 
of fluorescence and phosphorescence are sometimes 
classified under the more general term luminescence. 
Though there are comparatively few substances that 
exhibit the phenomenon to a marked degree, it is 
difficult to find materials that do not show the prop- 
erty slightly. This is readily seen by examining 
ordinary substances in an intense spectrum of the 



42 COLOR AND ITS APPLICATIONS 

light from a quartz mercury arc produced by means 
of a quartz optical system. In examining the fluor- 
escent light it is often convenient to look through a 
glass of such a color that it will transmit the fluor- 
escent light rays and absorb the exciting rays. The 
phenomenon is influenced by temperature, and in 
most cases the phosphorescence is temporarily in- 
creased in brightness by the application of heat or 
red and infrared rays, but the duration of this in- 
creased brightness is usually brief, as the phos- 
phorescence is rapidly extinguished by these agencies. 
The phenomenon is of great interest to the scientist 
and also in smaller degree to the colorist and light- 
producer. Fluorescent phenomena play a part in 
the appearance of certain colors (# 75), and there are 
possibilities for utilizing the phenomenon for the 
production of light for practical purposes. 

Some examples of fluorescence and phosphores- 
cence should be of interest. Sunlight and the light 
from carbon, mercury, zinc, iron, and silicon arcs 
are rich in ultra-violet rays which ordinarily are the 
most active in exciting fluorescence and phosphores- 
cence. In these cases it is convenient to remove 
most of the yellow, orange, red and infrared rays from 
the exciting beam by means of a dense violet glass 
and a water cell. Uviol blue glass is especially satis- 
factory. Water of a few centimeters depth is practi- 
cally opaque to infrared rays, but when pure is quite 
transparent to ultra-violet rays. The fluorescence, 
consisting in general of rays of longer wave-length 
than the exciting light, can be viewed through a glass 
of proper color without being confused by the color 
of the exciting light. Fluorescent materials are valu- 
able in visually investigating the ultra-violet region 
of a spectrum; for example, uranium glass is very 



THE PRODUCTION OF COLOR 43 

convenient for focussing a spectrograph for the in- 
visible ultra-violet rays. 

Aesculine, which is ordinarily transparent in solu- 
tion or in a gelatine film, fluoresces a bluish color 
under strong ultra-violet excitation. It is valuable in 
photography as a screen for absorbing ultra-violet rays. 
A solution of fluorescein or uranin is useful in demon- 
strating the path of light through various optical 
systems. It is best to prepare first a solution of 
moderate concentration and add this drop by drop 
to the tank of clear water. Of course the difference 
between the refractive index of the water and that 
of air must be taken into consideration when demon- 
strating the path of light through a given optical 
system immersed in water. Kerosene, an alcoholic 
solution of chlorophyl, anthracene, and many of the 
organic dyes exhibit the phenomenon of fluorescence. 
In cases of strong excitation the emission of light 
rays by some of these substances continues for some 
time after the exciting light is cut off. 

Substances that exhibit prolonged phosphorescence 
are chiefly the alkaline earth sulphides. Balmain's 
paint, an impure sulphide of calcium, is one of the 
most active and least expensive. Its phosphorescent 
light is of a bluish color. Other phosphorescent 
sulphides emit light of various colors, and by com- 
bining various ones, nearly any desirable color can 
be obtained. For demonstration purposes beautiful 
designs can be made by the use of various phos- 
phorescent media which emit light of different colors. 
The chief difficulties which limit the use of phos- 
phorescent substances are the scarcity of the exciting 
rays in ordinary light sources and the rapid decay 
of the intensity of the emitted light after the excitation 
has been cut* off. 



44 



COLOR AND ITS APPLICATIONS 



One instance of a commercial application of fluor- 
escence as a light source is the adaptation of a rhoda- 
mine reflector to the mercury vapor arc lamps by 
Peter Cooper Hewitt. The reflector consists of a 
white paper base upon which the rhodamine layer is 
placed. The latter is apparently protected by a trans- 
parent varnish. On focussing the spectrum from 
the quartz mercury arc upon it and viewing it through 
a red glass, it is seen that practically all rays from 
green to the extreme ultra-violet excite the red fluor- 



X 








?./^ 


F 




\ 








1 / 

/ / 


, "n I 






\ 




1 


/ / 




\ 
\ 

\ 








^^ 


^ 


/ S|^ 






40 0.45 


0.50 0.55 0.60 0.65 0.7C 










^. V^AWE 


LENGTH 







Fig.' 15. — Diagrammatic illustration of the action of the rhodamine fluorescent 

reflector. 

escence. That is, a photograph through a red filter 
of this projected spectrum appears quite the same as 
an actual spectrogram of the light from the quartz 
mercury arc. The action of the reflector is dia- 
grammatically shown in Fig. 15. The heavy vertical 
lines represent the visible lines of the mercury spec- 
trum, their lengths being approximately proportional 
to their energy intensities; curve R represents the 
reflection curve of the rhodamine dye, and F the 
fluorescent light. It is seen that a gap in the spec- 
trum of the light from a mercury arc equipped with 
this reflector exists in the blue-green region. While 
this reflector greatly improves the appearance of 
colored objects illuminated by the mercury arc, the 
light is still unsatisfactory for accurate color work, 
owing to the gap mentioned and to the sttong emission 




THE PRODUCTION OF COLOR 46 

lines. In Fig. 16 is given the spectrophotographic 
analysis, by means of a prism instrument, of the prop- 
erties of the rhodamine reflector. The slit of the 
spectrograph was purposely ad- 
justed somewhat wider than 
usual, on account of the long 
exposure (several hours) required 
to obtain a spectrogram of the 
fluorescent light, a represents 
the reflection of the reflector 
for tungsten light; b, the spec- 
trum of the tungsten light; [v b gyor 
c, the mercury spectrum; d, the uifravioieA v/s/bje 
reflection of the rhodamine re- ^''^i,^'^^^^^:^ 

flector illuminated by the total rhodamine fluorescent re- 
light from the mercury arc ; e, the 

mercury spectrum (shorter exposure); /, the isolated 
mercury green line produced by a special filter de- 
scribed later; ^, the fluorescence spectrum excited by 
the green line, the latter also appearing owing to diffuse 
reflection from the fluorescent reflector. This re- 
flector furnishes red rays to the mercury vapor lamp, 
when so equipped, at the expense of practically all the 
other rays. The scheme is an ingenious one, and 
probably paves the way for other practical uses of 
the phenomenon of fluorescence. 

In the study and practical use of fluorescent 
substances the solvent is of importance, owing to the 
influence upon the intensity of the fluorescence. 
Knoblauch ^ investigated the subject, obtaining the 
results given in Table III. The figures ranging from 
one to eleven indicate the order of the intensity, 
eleven being the most intense. The table is also 
of interest in suggesting a variety of solvents for dyes 
when these may be unknown to the experimenter. 



46 



COLOR AND ITS APPLICATIONS 



TABLE m 

Effect of Solvent upon the Intensity of Fluorescence 









-o 






"o 


















1 


i 


1 

•J3 


g 


•a 


-S 


1 


s 










S 


S 


6 


5 


2 


n 


*>, 


>% 


% 


1^ 


ii 


S 




3 


>> 
O 

3 


3 


3 

4 
5 


1 
3 
3 


1 

3 
6 
5 


1 

2 

4 
4 


n 




§• 


■§ 


& 


Aesculine 




Anthracene 


4 
91 




6 

1 


5 
1 


5 


B. Phenylnaphthylamin 






5 


1 








1 


« 






3 












Chrysoltn 


91 


3 


3 


3 




3 


1 












Eosine (sodium) 


1 


9, 


fi 


n 






4 




3 








Fluorescein (lithiutn) , , 


9, 


3 


n 


4 






1 












Fluorescene 






1 


2 
6 
9 
4 


4 
11 


9 


3 

10 
3 












PetroleuTTi 






3 




6 

4 


6 
3 
1 


B 


Phenosafranine 


1 


6 


7 
4 


9, 


Magdala red 






1 


Curcumin 






1 


2 

1 




3 


4 












Phenanthrene 














2 



























16. Useful Filters. — Very often in the study of 
color phenomena monochromatic light is desired, or 
ultra-violet and infrared regions of the spectrum must 
be isolated. For these reasons various filters which 
have been found convenient will be described. 

For obtaining intense monochromatic light the 
quartz mercury arc is a valuable source. Filters 
can be prepared for isolating the various lines. The 
filters can be made of dyed gelatine and cemented 
between glass plates (or quartz, if necessary) with 
Canada balsam, or the dyes can be dissolved in a 
proper solvent and used in glass or quartz cells. The 
filters can be tested visually by means of a spectro- 
scope or photographically with a panchromatic plate 
and a spectrograph. 



THE PRODUCTION OF COLOR 47 

For isolating the mercury yellow lines, 0.5790 and 
0.5764/1, chrysoidine and eosine are satisfactory. 

For isolating the green mercury line, 0.5461, 
neodymium ammonium nitrate and either potassium 
bichromate or eosine form an excellent combination. 
The former absorbs the yellow lines, and the latter the 
blue lines. Neptune green S and chrysoidine have 
been recommended for the purpose, but the author 
has found the former method more satisfactory. A 
cell of water will cut off the infrared rays satisfactorily 
for most cases when this is necessary. 

For isolating the blue line, 0.4359, cobalt blue 
glass and aesculine or sulphate of quinine form 
a satisfactory combination. The lines 0.4047 and 
0.4078 At can be practically isolated by a combination 
of methyl violet and sulphate of quinine in separate 
solutions. The line 0.3984 is transmitted to a slight 
extent. 

The ultra-violet line, 0.3650, can be practically 
isolated by methyl violet 4/? and nitrosodimenthyl 
aniline, methyl violet and acid green, or resorcine 
blue and aniline green. 

By the use of other line spectra and ruby glass 
or red dyes, monochromatic red light can be readily 
obtained. 

R. W. Wood has used a combination of strong 
cobalt-blue glass and a strong yellow, such as a satu- 
rated solution of bichromate of potash, for isolating 
the infrared rays beyond 0.69 /x. In photographs of 
a landscape through this filter the sky appears com- 
paratively black and the foliage white. 

Only the near infrared rays and no visible rays 
are transmitted by a solution of iodine in carbon 
bisulphide in a cell whose sides are composed of 
dense red glass. Wood has used lenses coated with 



48 COLOR AND ITS APPLICATIONS 

a thin film of chemically deposited silver for ultra- 
violet photography. Such a film is opaque to all rays 
excepting a narrow region in the vicinity of 0.32//. 
The formulae for silvering are readily found in any 
recipe book. 

Many of the organic dyes and colored glasses are 
useful as filters, depending upon the requirements. 
It is possible by a careful choice of filters and light 
sources to isolate any region of the spectrum desired. 
Photographic plates, owing to their diversity in spectral 
sensitivities, are valuable assets in some problems. 
Filters are often much more satisfactory in providing 
monochromatic illumination than the spectroscope, 
owing to the much greater intensities of radiation 
obtainable. 

An experiment which is very useful and educa- 
tive is the comparison of two yellows of different 
spectral compositions. A solution of potassium bi- 
chromate in water transmits yellow rays. If to a por- 
tion of this solution an aqueous 
solution of neodymium ammo- 
nium nitrate be added, the spec- 
tral yellow rays are no longer 
transmitted. The two solutions 
(c and dy Fig. 17) appear yellow 
in daylight. If not exactly of the 
same color, they can be readily 
brought to the same appearance 
by the use of more or less of 
the potassium bichromate in ^^^nimts'^of !hl s^a^l 

one of the solutions or by the hue but differing in spec- 

- -.^. - r J.1 11 tral character. 

addition of one of the yellow or 
orange dyes. It has been said that color depends 
upon the wave-length of light. However, color can 
not always be taken as an indication of wave-length. 




THE PRODUCTION OF COLOR 49 

because, as in this case, the two solutions appear of 
the same color — yellow; yet when examined by 
means of the spectroscope one (d) is found to trans- 
mit green, yellow, and red rays, while the other (c) 
transmits no yellow rays — only the green and the red 
rays. The latter is said to produce a subjective yel- 
low. The transmissions of the two solutions are shown 
in Fig. 17 compared with the spectrum of the mer- 
cury arc (g). It is seen that the absorption band of 
solution c falls in the same region as the yellow mer- 
cury lines, 0.5790m and 0.5764 /x, so that these yellow 
lines will not be transmitted by it. Therefore, since 
the yellow solutions are not transparent to the rays of 
shorter wave-length, the solution containing the neo- 
dymium ammonium nitrate (c) will, when illumi- 
nated by the mercury arc, only transmit the green line, 
0.5461, as shown in /. On viewing a mercury arc 
through each of the two solutions this is readily 
verified; one solution then appears a brilliant green, 
while the other remains yellow in color. The color 
of the green mercury line can readily be matched 
by a combination of colored glasses and dyes. The 
transmission of this combination filter is shown in 
e. The two yellow solutions c and d were made to 
match the yellow sodium lines, 0.5890 and 0.5896, 
which are shown (unresolved) in b. These lights, of 
the same color but of different spectral character, 
obtained with these solutions and properly chosen 
illuminants, were used in various interesting experi- 
ments to be discussed later (#37). 

An exhaustive spectrophotographic treatment of 
the transmission spectra of filters is outside the 
scope of this treatise, but a number of spectrograms 
of useful filters and ordinary glasses are given in 
Figs. 18 and 19. Uhler and Wood^ have prepared a 



^ 



U/trav/0/et %^ ^ 




Visible 



a. Tungsten Incandescent Lamp 

b. Carbon Arc 

c. Iron Arc 

d. Quartz Mercury Arc 




Green Ola ss (dense) 

Signal Blue (medium) 
Oreen(medium) 

Canary Yellow (light) 
Orange Gelatine (light) 
Green '* (light) 

Blue " (medium) 

I. Purple " (mediutm) 

m. Cobalt Blue Glass 

n. Clear Glass 

o. Iron Arc (bare) 



e. 

f. 

9- 
h 
I. 

J- 




Ultraviolet %:^ 



I 

Fig. 18. 



p. Signal Green (medium) 

q. Celluloid 

r. Cobalt ^lue Glass 

s. Clear Glass 

t. Canary Yellow (light) 
u. Quartz Mercury Arc 
V. Iron Arc 

visible 



Ultra-violet spectra, 
50 



Ultraviolet J, V/6/b/e 



'IIIIIIIIIMNt 



iiiiiinMiRi 



IlllllilffrtI 



1 r 



a. Quartz Mercury Arc 

b. Aesculine 

c. Acid Oreen -t- Ethyl VioJet 
d^ " 

e. Methyl Violet-hHitrosodimenthyl aniline 

f. Acid Green 

g. Aniline Oreen 
h. D/dtilled Wafer 
L Al rubidium 

J. Xylene Red 
k. Rosazeine 
I. Aniline Red 
m.Acid Green 

/?. Aniline Oreen 

0, Quartz Mercury Arc 

p. Aniline Yellow 

q. Fluorescein 

r, Tartrazine 

vS. Uranin 

t. Orange G 
u. Aurantia 
V. Quartz Mercury Arc- 






Ultraviolet %^Visible 



Fig. 19. — Ultra-violet spectra. 
51 



52 COLOR AND ITS APPLICATIONS 

valuable atlas of absorption spectra, and C. E. K. 
Mees ^ has prepared a similar treatise dealing with 
organic dyes. The spectrograms in Fig. 18 and those 
from / to V in Fig. 19 were photographed on Cramer 
spectrum plates, which are sensitive, but in varying 
degrees, to all visible and ultra-violet rays transmitted 
by quartz. Spectrograms a to h (Fig. 19) inclusive 
were made on an ordinary plate not appreciably sen- 
sitive to rays of longer wave-length than 0.48 /x. 

In Fig. 18, b, c, d, show the spectra of sources rich 
in ultra-violet rays. The next group, e to o inclusive, 
shows the transmission of common glasses for the 
radiation from an iron arc. The last group shows the 
transmission of some of the same glasses for the 
radiation from the quartz mercury arc. 

In Fig. 19 are shown the transmissions of various 
special screens. These screens are all cemented 
between two polished glass plates, each one-eighth 
inch in thickness. The glass is transparent to rays 
as short as 0.350^, but begins to absorb at this point, 
becoming practically opaque to ultra-violet rays shorter 
than 0.300 /x. Some of the first spectrograms illustrate 
the ability of specially prepared filters to isolate a 
narrow region of the ultra-violet spectrum at 0.365/>t. 
For instance, acid-green transmits this ultra-violet 
line, but also transmits much visible light not shown 
in the spectrogram on account of the scarcity of 
the transmitted visible rays in the spectrum of the 
mercury arc. However, by combining with this screen 
a visual complimentary dye, such as ethyl violet (a 
purple), which likewise transmits line 0.365^ but 
practically none of the visible rays transmitted by 
the acid-green, a visually opaque screen is produced 
which transmits rays near 0.365 m quite readily. In 
much the same manner combination screens are 



THE PRODUCTION OF COLOR 53 

devised for isolating any region of the spectrum. 
There is no limit to the number of screens that may 
be combined. The author has at times found it 
necessary to use as many as five dyes in combination 
to obtain the desired results. In Fig. 19, p to u 
inclusive show the rays in the quartz mercury arc 
radiation transmitted by six different yellow screens 
between glass plates. These screens appear of about 
the same color and transparency in daylight. AH 
are seen to transmit the yellow and green mercury 
lines, but three of them also transmit ultra-violet rays. 
Data regarding other media and the relative exposures 
and transparencies to tungsten light have been pre- 
sented elsewhere. 1^ The original negatives are. of 
course more satisfactory than the prints, because some 
of the fine detail is unavoidably lost in reproduction. 
These few specimen spectrograms have been inserted, 
not only on account of the interest in these special 
cases, but also as a means of giving some idea of 
the various details to be considered in the examina- 
tion and production of special screens to those who 
may not be famiHar with the procedjire. 

REFERENCES 

1. Phil. Mag. 12, p. 81. 

2. Recent Researches, p. 47. 

3. Trans. Roy. Soc. A, 203, p. 385. 

4. Ann. d. Phys. IV, 1908, 25, p. 377. 

5. Nature, Nov. 29, 1894, p. 113. 

6. Color, p. 194. 

7. Ann. d. Phys. 1895, 54, p. 193. 

8. Carnegie Inst, of Washington. 

9. Atlas of Absorption Spectra. 
10. Trans. I. E. S. 9, p. 472. 



CHAPTER III 
COLOR-MIXTURE 

17. That there is a tremendous variety of colors 
present in Nature can hardly escape the most in- 
different observer. A glance at a modern painting 
reveals the same abundance of tints and shades of 
color created by the hand of the artist from a few 
well-chosen fundamental colors. The artist mixes 
colors in a qualitative manner. He sometimes 
begins painting with some knowledge of the science 
of color-mixture, but after all his knowledge of mixing 
colors is largely qualitative and based upon asso- 
ciation with his stock of pigments rather than upon a 
knowledge of quantitative mixture of spectral colors. 
His success lies largely in a thorough acquaintance 
with the tools at his disposal, which are his pigments, 
yet an acquaintance with the science of color is of 
incalculable value to him, for the experimental results 
of the scientific study of color-mixture have largely 
formed the foundation of pure and applied art as 
well as of modern color theories. 

\JL8. Subtractive Method. — There are two dis- 
tinct methods of mixing colors; by addition and by 
subtraction of light rays. In a sense, color, as we 
ordinarily encounter it, is produced primarily by sub- 
traction (#12). That is, a fabric appears colored as 
a rule because the chemical used in staining it has 
the property of absorbing certain visible rays and 
of reflecting (or transmitting) the remaining rays. 
This subtraction of colored rays from white light 

64 



COLOR-MIXTURE 55 



results in the residual colored light. The integral 
color of the light absorbed is said to be comple- 
mentary to the color of the light remaining if the 
total light in the beginning were white light, say noon 
•sunlight. Of course in the foregoing case the ab- 
sorbed color has disappeared, so there is no oppor- 
tunity to view the complementaries. Any pair of 
complementary colors can be readily viewed by a 
comparatively simple apparatus. By means of a 
prism the spectrum of sunlight is produced at some 
point in space. A portion of this spectrum can be 
deflected from the original path by means of a prism 
of slight angle. The rays in each beam can be com- 
bined upon adjacent spots of a white surface by 
means of lenses, with the result that instead of a 
spot of white light two adjacent spots of colored light 
are seen. These two colored lights are obviously 
complementary, for if they are made to overlap they 
will be found to produce, by addition, a white light. 
By separating various portions of the spectrum all 
the pairs of complementary colors are readily pre- 
sented to view. As will be shown later, white light 
can be matched by mixing certain pairs of (and also 
by mixing three or more) spectral colors. This can 
readily be demonstrated by means of variable slits 
cut in a cardboard screen and held in front of the 
spectrum. If the slits have been placed in their 
proper position in the spectrum and properly adjusted 
in width, white light will result when the rays from 
these slits are combined on a white screen by means 
of a lens. 

\_The subtractive primary colors have been termed 
red, yellow, and blue. In reality they would be more 
exactly described as purple, yellow, and blue-green. 
They are the complementaries of the additive pri- 



56 COLOR AND ITS APPLICATIONS 

maries, as will be seen later. Some may prefer to use 
the term *pink' or * magenta' instead of * purple', but 
the hue is a purple consisting of red and blue. The 
tri-color processes of printing and color photography 
are based upon the subtractive principle of mixing 
colors. 

The principle of the subtractive method is well 
demonstrated by Fig. 20 (Plate II). If the three 
subtractive primaries, purple, yellow, and blue-green, 
are carefully made by the use of transparent media, 
water colors or printing inks, and are superposed, 
the results shown in Fig. 20 are obtained. First 
let us take a simple case of a yellow pigment on a 
white surface. The light passes through the colored 
film and is reflected back through it by the white 
surface. As the light passes through the yellow pig- 
ment it is robbed of the violet and blue rays, there- 
fore the light which reaches the eye is white minus 
violet and blue rays, and produces a sensation of 
yellow. In the processes of painting and color print- 
ing the three disks may be assumed to be micro- 
scopic in size, each being a minute flake of pigment. 
If two flakes be superposed, a yellow above a blue- 
green, a green color is obtained. The yellow flake 
does not transmit blue rays, therefore the green rays 
are the only remaining rays that will be transmitted 
by the blue-green pigment. These will be reflected 
by the white surface, and will pass again through the 
blue-green and yellow pigments, undergoing further 
changes tending to purify them, so that only green rays 
reach the eye. If the blue-green flake is above the 
yellow flake, the explanation must be reversed, but 
with the same result. The blue-green flake trans- 
mits blue and green rays; however, the yellow flake 
does not transmit blue rays. Therefore, only the 



COLOR-MIXTURE 57 



green rays will eventually be reflected to the eye. 
In the same manner the blue of the purple is sub- 
tracted by the yellow flake, and as purple consists of 
red and blue rays only, the red rays remain to be 
reflected to the eye. Therefore, yellow and purple 
flakes superposed produce red. Likewise the blue- 
green flake does not transmit red light, so that super- 
position of blue-green and purple flakes results in 
blue light being reflected to the eye. It is further 
seen that the superposition of the three subtrac- 
tive primaries results in a total extinction of light 
and black is the result. For instance, where the 
yellow and purple disks overlap, red results. The 
blue-green disk does not transmit red rays, so where 
it overlaps the red disk a total extinction results. 

Much interesting information may be obtained 
by carefully studying Fig. 20. Strips of colored 
gelatine laid over each other in checkerboard fashion 
present many striking examples of the subtractive 
method of mixing colors. In ordinary artificial light, 
screens made of ethyl violet (purple), uranin or 
aniline yellow, and filter blue-green, are excellent 
dyes for making the subtractive primaries for demon- 
strating the foregoing by superposition. Ethyl violet 
and naphthol green are practically complementary, so 
that when superposed no light rays are transmitted. 

19. Additive Method. — As already indicated, 
there are two distinct methods of mixing color, — 
the additive and subtractive, — but close investiga- 
tion often reveals both processes entering into some 
part of the production of color. The additive method 
always tends toward the production of white, whereas 
the subtractive method tends toward the production 
of black. The additive primaries are red, green, and 
blue. Some prefer to use the term * violet' instead 



58 COLOR AND ITS APPLICATIONS 

of * blue.' Blue, however, appears satisfactory and is 
a safer term than violet, because there are a great 
many who apply the term violet to purples. 

Long ago it was demonstrated th^t, by proper 
mixtures of the three well-chosen primary colors, 
any color can be matched. This is largely due to 
the fact that the eye is a synthetic rather than an 
analytic instrument. In Fig. 21 (Plate II), are illus- 
trated the principles of color-mixture by the additive 
method. It is seen that red added to green produces 
yellow; and further, when blue is added to this com- 
bination white is produced. In other words, yellow 
and blue mixed by addition produce white. It is well 
known, however, that yellow and blue (in reality 
blue-green) pigments when mixed by the subtractive 
method, as is done in painting and color printing, 
produce green. This is a much confused point, but 
is very simply explained when the character of the 
procedure of mixture is analyzed. Red and blue 
when added produce purple; and blue and green 
produce blue-green. It is to be noted that combina- 
tions of two of the additive primaries produce the 
subtractive primaries and vice versa. The additive 
method can be readily demonstrated by the use of 
colored lights projected upon a white surface. Prop- 
erly selected color-screens are necessary, but can be 
readily made from aniline dyes by carefully mixing 
them. It is difficult to describe the procedure quan- 
titatively, but there is no difficulty in producing the 
proper colors. 

Owing to the very unsatisfactory state of color 
terminology, it is impossible to present an accurate 
and definite list of complementary hues. However, 
a few complementaries are given in Table IV. 



COLOR-MIXTURE 



59 



TABLE IV 
Complementary Hues 

Red Blue-green (Cyan blue) 

Orange-red Green-blue (bluish cyan'i 

Orange Blue 

Yellow Blue-violet 

Yellow-green Violet-purple 

Green Purple (magenta) 



Wave-length of Complementary Spectral Hues 


0.6562 /A 0.4921 ft 


.5671 /A .4645 fi 


.6077 .4897 


.5644 .4618 


.5853 .4854 


.5636 .4330 


.5739 .4821 





An excellent scheme for showing the comple- 
mentaries is to arrange the spectrum around the 
circumference of a circle filling a gap between the 
ends of the spectrum, violet and red, with a series 
of purples from bluish purple to reddish purple. This 
has been called a color wheel, and is diagrammatically 
shown in Fig. 22. Here 
yellow and violet are shown 
as complementary. This may 
appear inconsistent with the 
foregoing discussion, but it 
will be noted that the terms 
*blue' and * violet' (as well 
as other color names) are 
indefinite. The term *blue' 
will always mean a spectral 
blue, but when used as a 
primary color its hue is defi- 
nite, whether the term stands 

for blue, violet, or blue-violet. If the complementaries 
have been correctly applied to the color wheel, a 
neutral gray should be obtained when it is rapidly 
rotated. 




Fig. 22.— The color-wheel for show- 
ing complementary hues. 



60 COLOR AND ITS APPLICATIONS 

20. Juxtapositional Method, — If a color be 
broken up into its component colors and the latter be 
applied in small dots with the point of a brush, the 
sensation of the original color will be obtained if it 
be viewed from a distance at which the eye is un- 
able to resolve the individual dots and providing the 
relative areas covered by the various colored dots 
are correctly balanced. Colors, excepting those en- 
countered in the spectrum, are usually far from mono- 
chromatic (#12), (Figs. 122, 123). For instance, a 
colored fabric which may appear a pure red will be 
found to reflect rays throughout considerable range 
of wave-lengths. If these component colors be repre- 
sented as pure as possible in minute dots of proper 
relative amounts, the foregoing result is readily 
obtained. For instance, if one end of a pack of cards 
be painted red and the other end green, on revers- 
ing every other card and viewing an end of the pack 
at a distance of several feet, it will appear yellow in 
color. The brightness apart from hue will be an 
average brightness. Many interesting experiments 
can be performed by ruling alternate fine lines of 
different colors on paper or on glass. For instance, 
purple and green lines alternated on paper will, if 
well chosen, produce an appearance of gray at some 
distance. Such a method of breaking a composite 
color into more nearly monochromatic components 
and applying the latter in the form of minute dots 
is the foundation of the principle of impressionistic 
painting. The processes of color photography devised 
by Joly, Lumiere and others are also based on this 
principle. 

21. Simple Apparatus for Mixing Colors. — 
There are very elaborate color-mixing instruments 
on the market for the purpose of demonstrating th^e 



COLOR-MIXTURE 61 



theory and practise of color-mixture. Apparatus that 
deals with spectral colors is as a rule the most sat- 
isfactory for accurate study and demonstration. How- 
ever, inasmuch as the colors ordinarily available in 
practise are far from monochromatic, that is, far from 
spectral purity, there is much virtue in the simpler 
forms of apparatus that can be made at small expense. 
In fact, for the foregoing reason the results obtained 
with some of the simpler instruments for demon- 
stration are more readily interpreted and applicable 
in practise than those obtained with apparatus dealing 
with pure spectral colors. 

Maxwell's disks offer a ready means for mixing 
colors. A shaft is arranged so as to be revolved at 
high speed. Colors painted on a disk can thus be 
mixed by rotating it at a high speed owing to per- 
sistence of vision. Light sensations do not reach 
their full value immediately upon application of the 
stimulus, nor do they decay to zero immediately upon 
the cessation of the stimulus. An infinite number of 
mixtures of pigments, including black and white, 
can be made with such a simple disk. Colored 
papers cut in circles and slit along one of the radii 
can thus be overlapped to any degree, and by the 
use of circles of various sizes a number of mixtures 
can be produced upon the same disk. This method 
is not truly an additive one, excepting in the addition 
of hues. The brightness is the mean of the sepa- 
rate brightnesses, each weighted by its angular 
extent. In Fig. 23 are typical color disks for mixing 
colors to produce grays. In I and III are repre- 
sented pairs of complementary colors respectively, 
yellow and blue, and green and purple. The inner 
circle consists of black and white, which can be varied 
in angular amounts to produce a neutral gray to 



62 



COLOR AND ITS APPLICATIONS 



match the gray produced by the addition of the two 
hues. In II are represented the three primary colors 
which when mixed by rotation produce a neutral 
gray which is readily matched by means of the inner 
black and white disks. These matches made under 
one illuminant will not ordinarily remain matches 
under another illuminant. Much of the early work 
in the science of color was done by means of rotating 
disks and even today they are extremely valuable 
in some investigations. The disks represented in 
Fig. 23 can be readily made from Zimmerman's 





Fig. 23. — Maxwell disks. 

colored papers. These papers are indicated in the 
catalogue by the letters of the alphabet and are given 
herewith as used in the disks already described. 
Yellow is designated as ^, blue as o, green as z, red 
as by and purple as a. For the black and white sectors 
any neutral tint papers with dull finish are satis- 
factory for producing the grays. 

The additive and subtractive methods as illus- 
trated in Figs. 20 and 21 (Plate II) can readily be 
demonstrated in permanent charts. The imported 
colored papers have been found satisfactory, owing 
to their comparative purity and unglazed surfaces. 
For demonstrating the subtractive method by the 
three overlapping disks the six colors and black are 
surrounded with a white background. The Zimmer- 



COLOR-MIXTURE 63 



man colors, designated by a, ^, Z, &, z, and o, may be 
used respectively for purple, yellow, blue-green, red, 
green, and blue. These six colors, with black and 
white, are sufficient for the construction of charts for 
the additive and subtractive methods. For demon- 
strating the additive method the three disks should 
be surrounded with black background, but in the 
case of the subtractive method the background should 
be white. For demonstrating these two methods 
of color-mixture with artificial light by means of 
transparent media, purple and green are readily 
produced by using gelatines dyed with ethyl violet 
and naphthol green respectively. When these two 
colored gelatines are superposed in proper densities 
of coloring, no light is transmitted. When light is 
passed through these media in juxtaposition in proper 
relative amounts and combined on a neutral tint 
diffusing surface a white light is produced. These 
two colors afford an excellent example of comple- 
mentary colors when used with artificial light. In 
daylight the ethyl violet screen appears deep blue in 
color, instead of appearing purple as it does in the 
light from a tungsten incandescent lamp. Other 
transparent media for further demonstrating these 
methods are readily selected from the many organic 
dyes available. Uranin, fluorescein, carmine, patent 
blue, and filter blue-green are satisfactory. 

In Fig. 24 the construction of an erratic color- 
mixing disk is illustrated. To a disk of stiff card- 
board a sectored disk of cardboard is rigidly fastened 
by means of a circular rivet. The latter disk has 
two 60° openings, as shown in a. Another disk, 
arranged concentric with the other disks and between 
them, is permitted to slip at will about the rivet as 
an axis. If the latter disk is prepared as shown in 



64 



COLOR AND ITS APPLICATIONS 



b many striking colors are obtained on rotating the 
combination. 

Fig. 25 illustrates a simple arrangement for color- 
mixing. The wheel is similar to that employed in the 
Simmance-Abady flicker photometer. The periphery 





Fig. 24. — An erratic color-mixing disk. 

of this wheel consists of truncated cones pointing 
in opposite directions. The axes of the cones 
are eccentrically placed at equal distances on either 
side of the axis of the wheel and parallel to it. 
In the angular position shown in the illustration, 

S s 






L' 



Fig. 25. — A simple color-mixer. 

the eye, looking at the wheel in a direction at right 
angles to the axis, sees one conical surface illumi- 
nated by one light, L, and the other by the other light, 
L\ Colored screens, SS, axe interposed between 
the wheel and the light sources. By having the lamps 
movable on a track any combination of brightnesses 
of two colors from transmitting media can be mixed 



COLOR-MIXTURE 



65 



by rotating the wheel rapidly. Pigments may also 
be applied directly to the wheel. 

In Fig. 26 is illustrated 
another simple instrument 
for mixing the colors from 
either opaque or trans- 
parent media. A wooden 
box is constructed as 
shown and painted black 
inside. G is a transparent 
plate glass and OO are 
ground opal glasses free 
from color. In mixing the 
colors of two transparent 
media, CC, the lamp, L, 
is moved to and fro on its 
track. Thus any propor- 
tions of the two colors can 
be mixed. If the colors 

of two opaque substances are to be mixed, 
and 00 are removed, the colored objects 




Fig. 26. — A simple color-mixer for 
transparent or opaque media. 



C/ 



/ 



/ 



/ 



/ 



/ 






/ 



/ 



/ 



/ 



fc/e 



\ 



\ 



\ 



\ 



\ 



\ 



\ 



\C 



Fig. 27. — Lambert's color-mixer. 



CC 

are 

placed at PP, and the 
lamp is moved to and 
fro as before. The 
range of mixtures in 
the last case is not 
infinite as in the case 
of transparent media; 
however, modifica- 
tions can readily be 

made so that 
— the range is 

extended. 



A simple experiment devised by Lambert, though 
not having the flexibility of the foregoing instruments, 



66 



COLOR AND ITS APPLICATIONS 



is of interest owing to its extreme simplicity. It is 
illustrated in Fig. 27. G is a plate glass and CC are 
colored objects. The colors are mixed one by re- 
flection, the other by transmission. By turning the 
glass and shifting the eye the proportions can be 
altered considerably. 

An apparatus of considerable use is a booth con- 
taining red, green, and blue incandescent lamps con- 
trolled by rheostats. If the colors are carefully made 
many interesting experiments can be performed, in- 
cluding the effect of quality or spectral character of 
light upon colored objects (#67). 

Many instructive experiments can be produced 
by the use of shadows cast by colored lights. One of 

especial interest is shown 
in Fig. 28, because it pre- 
sents the additive prima- 
ries, their complementaries 
(the subtractive primaries) 
and white light produced 
by the sum of the three 
primary colors, — red, 
green, and blue. In the 
middle of a circle of white 
diffusing blotting paper 
stiffened by a board are 
erected three planes of 
white diffusing material about eight inches in height. 
The latter meet at the center of the circle at 
angles of 120 degrees with each other. At points 
several feet away, along the three arrows, red, green, 
and blue lights are placed somewhat above the plane 
of the circle. These should be small sources and 
quite powerful, concentrated tungsten filament lamps 
being quite satisfactory. The experiment is best 




Fig. 28. — A shadow demonstration of 
the additive and subtractive methods 
of color-mixture. 



COLOR-MIXTURE 67 



seen if the plane of the circle is vertical. It will be 
seen that the nearly rhombic areas on the circle 
indicated by i?, G, and B each receive light from only 
one source. These areas will then appear respectively 
red, green, and blue. The areas on the opposite 
sides of the circle, 5G, P, and 7, each receive light 
from only two sources. They appear in colors com- 
plementary to the above primaries. They also rep- 
resent the subtractive primaries and the colors which 
remain after red, green and blue are subtracted 
respectively from three white lights. The remaining 
areas of the circle marked W represent the regions 
which receive light from each of the three sources, 
with the result that if the colors and intensities of 
the light sources are correct, and if the sources are 
sufficiently distant in comparison with the size of 
the circle, these areas appear a uniform white. This 
experiment is simple and is very satisfactory for dem- 
onstration before large audiences. The lights should 
be controlled by separate switches and rheostats. 

A rotating disk can be readily colored, so that it 
will appear, when viewed through a radial slit placed 
close to it, a fair approximation to the spectrum. The 
mixing of the colors by rotation obviates the neces- 
sity of the great care in blending colors in painting 
a spectrum that is to be viewed when stationary. 
The colors will not be of spectral purity, owing to 
the limitations of the pigments, but the disk will be 
instructive and affords a ready means of producing 
a spectrum for reproduction by color photography. 
An approximation to the prismatic spectrum can be 
readily produced as shown in Fig. 29. The approxi- 
mation can be made as close as desired by touching 
up various points with pigments where necessary 
or by varying the geometric figures. If a circle be cir- 



68 



COLOR AND ITS APPLICATIONS 



cumscribed about the inner square and a square in 
turn be circumscribed about this circle, and so on 

until four circles and 

four squares are pro- 
duced, the skeleton of 
the figure is ready for 
coloring. If the inner 
square be painted 
black and the succeed- 
ing hollow squares be 
painted red, yellow, 
green, and violet re- 
spectively and the re- 
mainder of the outer 
circle be painted black, 
a fair approximation 
to a prismatic spec- 
The same results can be pro- 




Fig. 29. — Illustrating a disk for approxima 
ting a prismatic spectnun. 



trum is obtained, 
duced with more difficulty with only three colors 
— red, green, and blue. If the spectrum pro- 
duced by rotation is not satisfactory at all points, it 
can be readily made so by the judicious use of 
pigments, or, as already stated, by altering the geo- 
metric figures. 

The more simple methods of mixing colors have 
been described in this chapter; however, it will be 
borne in mind that many of the instruments and 
methods considered in succeeding chapters are di- 
rectly or indirectly applicable to color-mixture. 



REFERENCES 

Captain W. de W. Abney, Colour Measurements and Mixture, 
London, 1891. 

O. N. Rood, Colour, 1904. 

Chevreul, Harmony and Contrast of Colours, 1839. 



CHAPTER IV 

COLOR TERMINOLOGY 

22. Hue, Saturation^ and Brightness. — One of the 
greatest needs in the art and science of color is a 
standardization of the terms used in describing the 
quality of colors and an accurate system of color 
notation. The term * color' in its general sense, is 
really synonymous to the term 'light.* It is used here 
by preference because it implies the consideration 
of the appearance of a surface or material object. 
The spectrophotometer is the most analytic instru- 
ment for examining colors (#26). By means of it the 
amounts of light of all wave-lengths reflected (or 
transmitted) by a colored medium may be obtained. 
These data are plotted in the form of curves shown 
by the dashed lines in Fig. 12. The full line curves 
represent the reflection (or transmission) coefficients 
of the pigments for energy for various wave-lengths. 
If the region under one of the curves indicated by a 
dashed line be integrated and this area compared 
with that obtained with the same illuminant for 
a white diffusing surface of known total reflecting 
power, the relative brightness of the colored medium 
under this illumination is obtainable. The domi- 
nant hue which is discussed below may be usually 
approximately determined by inspection of the curve, 
although in many cases it is impossible to estimate 
the dominant hue in this manner. It is thus seen that 
although the spectrophotometer is a valuable instru- 
ment for analyzing colors, there are further require- 

69 



70 COLOR AND ITS APPLICATIONS 

ments in color work better met by other instruments 
(Chapter V). 

The quality of any color can be accurately de- 
scribed by determining its hue, saturation or purity, 
and its brightness. (The latter term is analogous 
to the term * value' as used by the artist.) In the 
broadest sense, white, gray, and black are here con- 
sidered as colors, and a mere change in brightness 
alone is considered as a change in color. It appears 
necessary to assume this broad definition of color, 
inasmuch as brightness is distinctly one of the 
products of color analysis. Hue is suggested in 
the name applied to the color. The dominant hues 
of most colors are accurately represented by spectral 
colors; however, there are composite colors, — the 
purples, which consist of red and violet, for which no 
spectral colors are found to represent their hues. In 
these cases it is satisfactory to determine the domi- 
nant hue of the complementary colors. The satu- 
ration or purity is a measure of the relative amount 
of white light in the color. In other words, all colors 
excepting purples can be matched by diluting spectral 
light of a definite wave-length with white light. The 
greater the percentage of white light required in the 
mixtures, the less saturated the colors are said to 
be. The brightness of a color can be found by com- 
paring it by means of a photometer with a surface 
of known brightness. It is well to note that in the 
analysis of a color its absolute brightness is measured 
by comparing it with a brightness of known value. 
Inasmuch as its brightness depends upon the inten- 
sity of illumination of a given spectral character, its 
reflection coefiicient for a standard white light should 
be determined in order to compare it with other 
colors in this respect. This latter measurement in- 



COLOR TERMINOLOGY 



71 



volves all the difficulties of color-photometry treated 
in Chapter IX. 

There is much confusion in the application of the 
terms *tint,' *tone,' * shade,' 'intensity,' etc. Many 
use these terms wholly unjustifiably. It is true that 
the final usage is somewhat a matter of choice at the 
present time, but the terminology adopted here appears 
to the author to be consistent with other nomen- 
clature adopted by the physicist, photometrist, and 




Q b 

Fig. 30. — Disk *a,' for varying only the saturation of a color. 
— Disk * b,' for varying only the brightness of a color. 

lighting expert, and best justified by usage and the 
dictates of common sense. On diluting a color with 
white light, tints are obtained; that is, tints are un- 
saturated colors. By the admixture of black to a 
color (in effect the same as reducing the intensity of 
illumination) the brightness is diminished without 
altering either the hue or the saturation, and various 
shades are produced. Only the relative brightnesses 
of shades are usually of interest, although for obtain- 
ing a basis of notation it may be desirable to deter- 
mine their absolute values. In a. Fig. 30, is shown a 
simple means of varying the saturation of a color 
without altering either the hue or brightness. On a 



72 COLOR AND ITS APPLICATIONS 

circle of colored paper is glued a gray paper of the 
same brightness for the given illumination and of 
the form shown by the shaded area. On rotating 
the disk this gray will be mixed in various angular 
proportions from 360 deg. to deg. The gray paper, 
having been selected of the same brightness as the 
colored paper under the illuminant used in the experi- 
ment, does not alter the brightness upon mixing 
the two components by rotation; being non-selective 
in its reflection it does not alter the hue. Thus 
various degrees of saturation of the original color 
are obtained. 

The brightness can be varied, as shown in b, Fig. 
30, without altering either the hue or saturation by 
fastening to the original circle of colored paper a 
black paper cut in the same form as the gray paper 
shown in a. If the paper were perfectly black it is 
seen that it cannot alter either the hue or satura- 
tion. As a matter of fact no available black papers 
are totally non-reflecting, so that some light is added 
to the color. This can be reduced to a minimum, 
however, by the use of a hole in a deep velvet-lined 
box. In this case the black sectors shown in b would 
be replaced by openings of the same contour in the 
disk. For convenience of construction the areas 
occupied by the black and colored papers may be 
reversed. In this connection it is well to emphasize 
that ordinary black surfaces are far from totally 
absorbing. This can readily be demonstrated by 
a box open at one end lined with black velvet. Over 
the open end place a black cardboard with an opening in 
it and it will be seen that the opening will appear very 
much darker than the black surface surrounding it. The 
foregoing demonstration may be easily performed by 
varying the brightness of a colored paper relative to that 



COLOR TERMINOLOGY 



73 



of a paper of the same color which surrounds it, by vary- 
ing the intensity of the illumination of the patch at the 
same time maintaining the absolute brightness of the 
surroundings constant. 

Instruments have been designed for the analysis of 
color quality into the three component factors, hue, 
saturation, and brightness. These are treated in #27. 

23. Tri-color Method. — It is well known that 
any color can be matched by combining the three 
primary colors, red, green, and blue, in proper pro- 
portions. Many instruments have been devised for 
this purpose, the most elementary being the Maxwell 
disks, and the more elaborate and accurate are those 
employing spectral colors. The results of such a 
method are expressed mathematically in the equation 
xR+yG + zB = C, where the values of jc, ^, and z 
are the fractional parts of the red, green, and blue 
lights, respectively, that must be combined to match 
the color, C. This method has limitations because 
it does not give the results directly in terms of hue, 
saturation, and bright- 
ness. Some of these in- 
struments, however, can 
readily be adapted to the 
measurement of the last 
two factors. In order to 
plot the values of jc, y, 
and z, it is necessary to 
employ tri-linear coordi- 
nates, there being three 
variables to be repre- 
sented. The results are 
readily represented in 
the Maxwell color triangle, illustrated in Fig. 31. The 
green component increases from zero at the base 




Fig. 31. — The Maxwell color-triangle. 



74 COLOR AND ITS APPLICATIONS 

line, RBj to 100 per cent at G. Likewise the red, 
Rj and green, G, components increase from zero at 
the base of the perpendiculars erected from the sides 
respectively opposite to their apexes. The data are 
plotted by erecting three perpendiculars proportional 
to the respective values of /?, G, and 5, starting at 
points in the opposite sides of the triangle respectively, 
such that the three perpendiculars intersect at the 
same point. Purples are found along the base line, 
RB, varying in the proportions of R and B from /? = 0, 
B = 100, to R= 100, B = 0. Yellows are found along 
RG and blue-greens along GB, White, which is 
usually represented by ^R-\- lG-\- ^B = W, is found 
at the center of the triangle. The curved line rep- 
resents the positions of the spectral colors in the 
color triangle; that is, each point on the curve rep- 
resents the primary sensation values of a particular 
spectral color. Some important lines of the spectra 
of cadmium and mercury are also shown. The less 
saturated colors are found near the center of the 
triangle and the more saturated ones near the sides. 
It is thus seen that spectral colors throughout a large 
range of wave-lengths arouse the three primary 
sensations, according to the Young-Helmholtz theory. 
(See #28, 47.) The primaries of course are found 
at the angles of the triangle. Complementaries are 
represented as being on opposite sides of the center 
of the triangle on a straight line passing through it. 
The dominant hue of a color is found by drawing a 
straight line from the center of the triangle through 
the point representing the color and continuing it 
until it intersects the curve representing the spectrum. 
The latter point of intersection represents the domi- 
nant hue of the color. The tri-color method in- 
volves the use of an invariable white light, that is 



COLOR TERMINOLOGY 



75 



1 



noon sunlight or its equivalent. A curve represent- 
ing spectral complementaries is shown in Fig. 32. 

Results obtained by 
this general method with 
different instruments are 
likely to vary considerably. 
This is due in part to vari- 
ations in the spectral char- 
acter of the white light 
standard, and also to the 
transmission characteris- 
tics of the color-screens 
used in these instruments 
not employing spectral 
primary colors. The primary sensation values of the 
screens should be determined and the measurements 
be given in sensation values (#28). The use of the 
plane triangle is limited to the plotting of the analyses 
of colors of equal brightness. In order to include 

^Q the brightness factor the figure 



\J.OKJ 
































' 
























































0.45 






1 






































































































An 























055 



0.60 
yd. WAVE LENGTH 



0.65 



Fig. 32. — spectral complementaries. 




takes the form of a solid inverted, 
pyramid, shown in Fig. 33. The 
various triangular planes parallel 
to the base represent planes for 
plotting colors of different bright- 
nesses. The apex represents 
black. A line joining the point, 
W (white), with the apex passes 
through a complete range of 
shades of white, that is, of grays. 
Along the dotted line from x to 
the apex are a series of colors 
of constant hue and saturation, 
but varying in brightness. The color pyramid has been 
modified in various rays to fit experimental results in- 



Black 
Fig. 33. — A color pyramid. 



76 



COLOR AND ITS APPLICATIONS 



volving physiological and psychological influences. One 
of these modifications from Titchener ^ is shown in Fig. 
34. At the two poles of this double pyramid are the 

extremes of white and black; 
upon the axis connecting the two 
poles are located the complete 
range of grays. Around the pe- 
riphery of the middle plane are 
located those colors of middle 
brightness and maximal saturation. 
Other points in the solid represent 
other colors of varying hue, satura- 
tion, and brightness. From the base 
toward white, tints are found; in 
the other direction shades are 
found. This pyramid, it will be 
noted, has four sides, the four 
angles representing red, yellow. 




Fig. 34. — The double pyra- 
mid. (After Titchener.) 



green, and blue. Obviously this 
solid does not directly represent 

color analyses as obtained by the tri-color method. 

Its significance will be better understood on referring 

to the Hering theory of color vision in #49. 
The tri-color method is 

discussed further in Chapter 

V. A simple means ^ of dem- 
onstrating the Maxwell color 

triangle in actual colors is 

illustrated in Fig. 35. A box 

6 inches in depth, and whose 

section forms an equilateral 

triangle about 18 inches on 

a side, is made of wood, 

with its back containing vent 

holes. A ground flashed-opal glass in 




Fig. 35. — A demonstration color 
triangle. 



the form 



COLOR TERMINOLOGY 77 

of an equilateral triangle somewhat smaller than 
the section of the box forms the front side. In 
the three corners of the box are placed respectively, 
red, green, and blue spherical-bulb, concentrated- 
filament tungsten lamps. After proper adjustments 
of the position and color of the lamps, the diffusing 
glass, which has its roughed side inward, assumes 
the colors of a color triangle. A close approximation 
can be approached, depending upon the care exer- 
cised in adjusting the position of the lamps and the 
distribution, color, and intensity of the light. Inter- 
esting demonstrations of retinal fatigue and after- 
images are readily made with this simple apparatus. 
For coloring the lamps ordinary colored lacquers are 
satisfactory if properly mixed to obtain the exact 
hues. The aniline dyes can be used with satis- 
faction. These colors are not permanent, but are 
sufficiently durable for such an apparatus. If the 
coloring is placed on separate plates of glass, it will 
remain unfaded for a long time with proper ventila- 
tion. 

24. Color Notation. — The need for a universal 
color notation is admirably illustrated by Munsell ^ 
in quoting from a letter by Robert Louis Stevenson, 
writing from Samoa to a friend in London, as follows : 

" Perhaps in the same way it might amuse you to send us any 
pattern of wall paper that might strike you as cheap, pretty and suit- 
able for a room in a hot and extremely bright climate. It should 
be borne in mind that our climate can be extremely dark too. Our 
sitting room is to be in varnished wood. The room I have particu- 
larly in mind is a sort of bed and sitting room, pretty large, lit on 
three sides, and the colour in favour of its proprietor at present is a 
topazy yellow. But then with what colour to relieve it? For a Uttle 
work-room of my own at the back I should rather like to see some 
patterns of unglossy — well 1^11 be hanged if I can describe this red — 
it's not Turkish and it*s not Roman and it's not Indian, but it seems 



78 COLOR AND ITS APPLICATIONS 

to partake of the two last and yet it can't be either of them because 
it ought to be able to go with vermillion. Ah, what a tangled web 
we weave — anyway, with what brains you have left, choose me and 
send me some — many — patterns of this exact shade." 

Here is a man accustomed to present his thoughts 
in writing in a clear manner, yet he acknowledges 
failure in his effort to describe colors and closes his 
letter with the request, perhaps a bit sarcastic, that 
he be sent "patterns of this exact shade." Other 
sciences have exact and practically universally ac- 
cepted terminology. Music has its well-developed 
notation, which is definite and descriptive, and quite 
universal in adoption, but there is no universal 
scheme of color notation. Colors are named in very 
inexact, unwieldy, and often totally non-descriptive 
terms. We have rose, Indian red, Alice blue, pea 
green, olive green, cerise, taupe, baby blue, Copen- 
hagen blue, king's blue, royal purple, invisible green, 
etc. Thus flowers, vegetables, cities, the savage 
and the royal family, are used to describe colors. Is 
there a more ridiculous instance of neglect.'* Those 
who work in color often find themselves helpless in 
describing colors to others. Surely a color notation 
based upon color science should be acceptable, even 
though somewhat empirical. Musical notation is 
somewhat arbitrary, yet it has met with almost uni- 
versal adoption. An acceptable color notation must 
involve the factors which influence the quality of a 
color, namely hue, saturation, and brightness. 

An attempt was made by Runge as early as 1810 
to build up a color notation by the use of a sphere 
with red, yellow, and blue, placed around the equator 
and separated from each other by 120 degrees, with 
white and black at opposite poles. Perhaps the 
greatest virtue in this attempt is the fact that it was 



COLOR TERMINOLOGY 79 

one of the early constructive efforts. Chevreul, 
whose work on the effect of simultaneous contrast of 
colors in the practical textile industry is well known, 
constructed a hollow cylinder built up of ten sections 
perpendicular to the axis. Around the upper section 
red, yellow, and blue were equally spaced. The 
lowest cylindrical section was black, and the eight 
intervening sections were graded from top to bottom 
by adding increasing amounts of black. Munsell 
criticises these attempts, on account of the yellow 
being very light and the blue being very dark, which 
makes impossible any coherency in the brightness 
scales of the three colors. Inasmuch as the bright- 
ness scale of the yellow in the Chevreul Qolor cylinder 
increases much more rapidly from the bottom toward 
the top than the brightness scales for blue and red, 
Munsell suggests that the yellow side of the cylinder 
be increased in length. This would result in the 
tilting of the sections more and more as the scale 
of brightness progressed from the bottom toward 
the top. Perhaps a general criticism for most of these 
schemes of color notation is that geometrical figures 
are chosen and the colors are made to fit. The 
latter method is perhaps partially justifiable from 
the standpoint of physical measurements. There 
is another viewpoint in considering a color notation, 
and that is from the standpoint of harmony of color. 
In this treatise we are not so much concerned with 
the latter viewpoint, but it is of interest to consider 
a system of color notation devised by Munsell from 
the standpoint of the use of color in painting. In 
describing a color by this system the initial of the 
name of the color indicates the hue, and numerals 
represent the saturation and brightness. For example 
R7 represents a color whose hue is red and whose 



80 COLOR AND ITS APPLICATIONS 

saturation and brightness are respectively 7 and 5. 
The brightness scale is divided into ten parts, and 
the degrees of saturation shown vary with the bright- 
ness. For simplicity ten hues are balanced around 
the equator of the sphere somewhat after the manner 
shown in Fig. 22. The lower pole of the sphere is 
black and corresponds to zero on the brightness scale. 
The upper pole is white and corresponds to brightness 
10 on the same scale. On slicing off a portion of 
the sphere through a plane corresponding to a certain 
brightness, various degrees of saturation are encoun- 
tered. The saturation decreases toward the center, 
the axis of the sphere consisting of a scale of gray 
S. However, the sphere does not completely satisfy 
Munsell. He therefore constructs a * color tree' so 
that varying numbers of steps in saturation can be 
represented, depending upon the hue and position 
in the brightness scale. The scheme is built up on 
the principle of the harmonious use of colors and in 
this respect departs somewhat from the scope of 
this book, which treats more with physical mixtures, 
regardless of the use of colors in harmony. The 
system is an interesting one and is the result of a 
noteworthy attempt to be freed from a state of color 
anarchy. 

MunselPs color tree is illustrated in simple form 
in Fig. 36. The base of the tree is black, the top 
white. In the small model illustrated three bright- 
ness levels are shown, namely 3, 5, and 7 in the 
arbitrary brightness (value) scale. The degrees of 
saturation shown vary with the * brightness level.' At 
* level 3' in the brightness scale blue is shown to the 
eighth degree of saturation. By the irregularity in 
the contour of the planes representing different 
brightness levels it is seen that the relative number 



COLOR TERMINOLOGY 



81 



of degrees of saturation shown for various colors 
depends upon the brightness level under considera- 
tion. At brightness level 3, PB (purple-blue) was 
shown with the highest degree of saturation, namely, 
8. At brightness level 5, R ranked first in degree of 
saturation, its highest being 10. At the brightness 
level 7, yellow was shown with the highest degree 

Wh/fe 



Ye/fov/ 



Yet low- red 



Red 




Red-purple 



Fig. 36. — The A. H. Munsell color tree. 

of saturation, namely 8. For a complete discussion 
of this system, the reader is referred to the original 
description by Munsell.^ 

Numerous scales have been devised involving, 
either separately or combined, the factors hue, 
saturation, and brightness. All of these assist in 
bringing order out of chaos, but they constitute only 
the first steps toward a comprehensive system of 
color notation. The hues are usually expressed by 
names of spectral colors and purple, but the bright- 
ness is seldom more definite than is found in such 
expressions as W, HL^ L, LL, Af, HD^ Z>, ZZ>, and 



82 



COLOR AND ITS APPLICATIONS 



By which represent white, high light, light, low light, 
medium, high dark, dark, low dark, and black re- 
spectively. Examples of such charts (devoid of color) 
are shown in Fig. 37. These were taken from Book 
VI of Prang's text-book of art education. Such 



NtUTRAL VALUe 






CHART -B 



CHART - C 



Fig. 37. — Prang*s color and brightness scales. 



charts should pave the way toward a final scientific 
color notation. 

Another system of color notation is shown in 
Fig. 38. This is used by Ruxton in the mixture of 




Fig. 38. — Ruxton's color mixture chart for printing inks. 

printing inks. The chart is printed in colors, there 
being 144 colors, varying in hue, saturation, and bright- 
ness. The terminology is somewhat different than 
used in this text. The 144 colors are obtained from 
six fundamental colors, namely red, orange, yellow, 
green, blue, and purple. These six colors are de- 
scribed as spectral colors, so it is likely that purple 



COLOR TERMINOLOGY 83 

is the name applied to a color meant to be violet. 
The starting point in obtaining the total array of 
colors is in the bottom row of Section 3. The larger 
rectangles represent the six fundamental colors, which 
are the purest or most saturated on the chart. The 
fundamental red is marked 820. The small square 
areas represent the intermediate hues and are ob- 
tained by mixing the fundamentals on either side. 
Red-orange for instance is obtained by mixing red 
and orange (820 and 840). The three horizontal 
rows above this row of twelve colors are made by 
adding white to the colors of the bottom row. Thus 
in the top row are found the least saturated colors 
in Section 3. Two degrees of saturation lie between 
the top and bottom rows. Thus in Section 3 there 
are 48 colors, six fundamental colors increased to 
twelve by mixing adjacent fundamentals (red and 
violet are mixed, producing 810) and these twelve 
colors decreased in saturation in three steps by the 
addition of white. Sections 1 and 2 are produced 
by adding black to the corresponding colors in Section 
3, thus reducing their brightness. In Section 1 are 
the colors of lowest brightness. These are named 
*hues' but in a different sense than that in which 
the term *hue' is employed here. They could be 
termed * Values' with better consistency. The *bi- 
hues' (bi-values) in Section 2 are obtained by 
mixing one part by weight of the colors in Section 3 
to one part of the corresponding color in Section 1. 
Thus it is seen that in a more correct sense there 
are twelve hues represented on the chart (bottom 
row in Section 3). With the hue and brightness 
constant saturation is found to be present in four 
degrees (moving vertically in Section 3). With the 
hue and saturation constant, the brightness is found 



84 COLOR AND ITS APPLICATIONS 

to be present in three values (moving from left to 
right, Sections 3, 2, 1, so-called * colors,' *bi-hues' 
and * hues'). Besides these there are 72 other colors 
in which brightness and saturation appear in six 
combinations for each of the 12 hues. In other 
words, in a broad sense there are present 144 colors 
made up of twelve hues by varying the brightness 
and saturation. Six of the twelve hues are made by 
mixture of the adjacent hues in the bottom row of 
large rectangles in Section 3. Each rectangle being 
numbered, the chart systematizes the mixture of 
printing inks. Such progress is commendable and 
highly desirable, even though empirical. 

There are many other methods, but these few 
have been cited to show the lack of standardization 
of color notation and to illustrate that a system 
however empirical is just as desirable for the de- 
scription of color as a system is for music notation. 
There is much yet to be done before a system of color 
notation is devised which will be universally adopted. 
First there should be some definite terms adopted 
descriptive of the factors influencing the quantity 
of a color, namely * hue,' * saturation,' ' brightness.' The 
term *hue' is used in a more definite sense than the 
terms applied to the two other factors. For satura- 
tion the terms * chroma,' * purity,' 'intensity,' and others 
are being used. For brightness the terms * luminos- 
ity,' * value,' * hues' or *bi-hues,' and others are 
being used. Purples are often called violets or reds. 
These are examples of usage from which general 
confusion arises. The problem of color terminology 
does not defy solution. As a matter of fact all the 
quantities involved in a scientific system of notation 
are readily measurable. Hue, saturation, and bright- 
ness are easily determined. The available hues. 



1 



COLOR TERMINOLOGY 86 

with the exception of purple, are invariable, consisting 
of the spectral hues. Scales of brightness (value) 
can be divided into any given number of parts and 
named in some consistent manner. The use of the 
terms *high light,' *low light,' * medium,' *low dark,' 
etc., is perhaps satisfactory, but the brightnesses that 
they represent should be standardized in absolute 
measurements in order to produce a universal scale 
of relative brightnesses. In fact all the terms re- 
quired in a satisfactory and scientific system of color 
notation can be measured for their absolute values. 
This would reduce the systems to one basis. Such 
a universal system must certainly be adopted even- 
tually, and those interested in color should put forth 
effort to hasten the day. 

REFERENCES 

1. Primer of Psychology, 1899, p. 41. 

2. Psych. Rev. 20, May, 1913. 

3. A Color Notation. 

OTHER REFERENCES 

Sir William Abney, Color Mixture and Measurement. 
p. N. Rood, Textbook on Color. 

J. G. Hagen, Various Scales for Color-Estimates, Astrophys. 
Jour. 1911, 34, p. 261. 

k. Zindler, Color Pyramid, Zeit. f . Psych. 1899, 20, p. 225. 
R. Ridgeway, Color Scales. 



CHAPTER V 

ANALYSIS OF COLOR 

25. The Spectroscope, — As already indicated, 
colors can be analyzed in various ways. The method 
adopted in a given case will naturally depend upon 
data desired. The spectroscope affords a simple 
means of examining colored light, but the results of 
the visual inspection are only qualitative. There 
are various designs of spectroscopes available, all 
based upon either the principle of the prism or of 
the diffraction grating (#8 and 9). An ordinary prism 
spectroscope can be converted into a direct-vision 
instrument by combining two prisms made from 
different kinds of glass, so that dispersion is obtained 
for a certain ray without deviation. Crown and 
flint glasses differ in refractive index (Fig. 8), hence 
if prisms of each of these two glasses be made of 
proper refractive angles and combined so that their 

j separate deviations prac- 

I /\f/\ (a I tically annul each other, 

I / V N \i I say, for the sodium line, 

Fig. 39. — A direct-vision prism spec- dispersion is produced 

*^°^^°P^- without deviation for this 

^ ray. Such a simple spec- 

troscope is shown in! Fig. 
1 39. A simple diffraction 

Fig. 40. — A simple grating spectro- grating SpOCtrOSCOpe Can 

^^°^®* be readily made, as shown 

in Fig. 40. A replica of a diffraction grating (#9) is 
placed between two pieces of plate glass at G. By 



I 



ANALYSIS OF COLOR 87 

placing a lens at L the instrument is considerably 
shortened, so that it can readily be made of a pocket 
size. These, the simplest forms of spectroscopes, 
are only useful for rough qualitative analysis of the 
spectral character of light emitted by light sources 
or transmitted or reflected by colored media. A 
convenient form of spectroscope for qualitative analy- 
sis is the comparison spectroscope. This contains 
two or three distinct optical systems, so that two or 
three spectra may be viewed in juxtaposition. Such 
an instrument might be considered as roughly quan- 
titative in its analyses, owing to the opportunity of 
estimating relative intensities of a given light ray 
in the two or three spectra. 

More elaborate spectrometers will not be con- 
sidered here, for the function of the spectrometer 
is for qualitative analysis. However, in this respect 
such instruments are of considerable value in color 
work. Photographic accessories are readily attached 
in place of the eyepiece. If an absorption wedge be 
placed before the slit, so that its transmission varies 
along the length of the slit, the spectrograms will 
roughly indicate the relative spectral distribution of 
energy, providing proper filters are used to allow for 
the variation in plate sensibility. Sometimes it is 
advantageous to compensate for the unequal spectral 
distribution of energy in the illuminant, especially 
in the examination of colored media. 

26. The Spectrophotometer, — This instrument 
consists in principle of two spectroscopes, arranged 
so that the intensity of rays of the same wave-length 
in the two spectra can be photometrically compared. 
The results obtained are quantitative. A diagram- 
matic sketch of the optical system of a spectro- 
photometer is shown in Fig. 41. Light enters the 



88 



COLOR AND ITS APPLICATIONS 






0-eE 



instrument from two sources at the slits S and S\ 
respectively. At L is a Lummer-Brodhun photometer 
cube so constructed that through a 
part of the* field light rays are trans- 
mitted directly from S' to the prism 
P and from the remainder of the 
field light rays from S are reflected 
toward the prism. After being dis- 
persed by the prism the colored rays 
pass on to the eye placed at T. The 
wave-length of these rays depends 
upon the angular position of the prism 
which can be rotated. The photom- 
eter field is similar to that viewed 
in an ordinary Lummer-Brodhun 
photometer. 
A small direct-vision comparison spectroscope is 
of considerable use in color work. Such an instru- 
ment designed by Nutting ^ contains a pair of Nicol 
prisms, NNy for altering the brightness of one of 
the spectra, as shown in Fig. 42. A right-angled 




Fig. 41. — The spectro- 
photometer. 



a^MI 



)( 



i 



Fig. 42. — The Nutting pocket spectrophotometer. 



prism, R, reflects light into the slit from one of the 
sources. The instrument, which is called a pocket 
spectrophotometer, in itself is merely for qualitative 
analysis. It can be set up permanently and used 
for quantitative measurements. However, in order 
to make such an instrument portable and compact, 
yet available for obtaining qualitative data, the author 
devised the attachments shown in Fig. 43. An 
attachment, i4, containing a miniature tungsten lamp 



ANALYSIS OF COLOR 



89 



which illuminates a ground flashed-opal glass, can 
be removed from its present position if desired and 
attached at B. When the comparison source in A 




Fig. 43. — A small portable spectrophotometer for quantitative analysis. 



is in the position shown the unknown source is placed 
at B. The sleeve, O, was placed on the instrument 
to support A. S controls the slit widths. A mi- 
crometer screw with a graduated scale and drum is 
attached at M to the slide containing the observing 
slit, C, which is moved across the image of the 
spectrum. The drum is calibrated in terms of wave- 
lengths and the scale of the revolving Nicol prism 
in terms of transmission of light. The current 
through the lamp is obtained from a battery controlled 
by a rheostat and is measured by means of a small 
ammeter. The range of the instrument can be 
extended by varying the current through the lamp. 
This photometric field is not as satisfactory as might 
be desired, for it consists of two narrow bands juxta- 
posed at their ends. The instrument is very small, 



90 



COLOR AND ITS APPLICATIONS 



less than 8 inches long, is mounted on a tripod, and 
is really portable. 

There are many designs of spectrophotometers, 
but all have the same object. It is necessary to 
be able to vary the luminous intensity of one-half 
of the photometer field. This is done by varying the 
position of one of the light sources, by the use of 
Nicol prisms, by a neutral tint absorbing wedge, or 

by sectored disks. A 

convenient means is the 
variable sectored disk 
developed by Hyde,^ a 
diagram of which is 
shown in Fig. 44. The 
disk is mounted upon a 
motor-driven shaft and 
arranged to be moved 
horizontally in its plane 
along the line CD in 
front of the slit S, by 
means of a micrometer 
screw. The transmission is nearly proportional to 
the lateral displacement. 

By means of the spectrophotometer, results can be 
obtained directly in terms of relative energy such as 
are plotted in Fig. 5 (Table II) and Figs. 122, 123. 
In this case the various rays in the unknown spectrum 
are compared directly with the corresponding rays in 
a spectrum of known distribution of energy. As has 
been previously stated the spectrophotometer is an 
analytical instrument, and by its use the spectral 
character of the light reflected or transmitted by 
colored media is readily obtained. An example of its 
use and a practical means of greatly reducing the 
number of readings is given below. During the 




Fig. 44. — The variable sectored disk. 
(After Hyde.) 



ANALYSIS OF COLOR 



91 



development of a glass which could be used with the 
tungsten lamp to produce artificial daylight ^ the 
procedure involved the examination of many glasses 
containing various proportions of coloring ingredients. 
A glass which proved unsatisfactory at the thickness 
at hand might be found satisfactory at another thick- 
ness. Therefore it was necessary to grind and polish 
the samples as they came from the glass factory into 
many different thicknesses or in the form of a wedge. 



1.00 
0.80 
0.70 
0.60 
0.50 
0.40 
0.50 

o OZO 

V) 

r 0.08 
< 0O7 

jl 005 

004 



005 
0.02 



6 

i ^-0 




0.466 
0A85 
0.508 
0.52 f 
0.536 
'0.544 
0.552 
0.570 



aseo 

0.612 

0.65Q 
0.668 



12 3 4 6 6 7 

THICKNESS OFGLASS IM MILLIMLTERS 



Fig. 45. — Scheme for reducing the amount of spectrophotometric work in exam- 
ining transparent colored media. 

This necessitated making a set of spectrophotometric 
readings for a considerable range of thicknesses. By 
utilizing the law relating transmission and thickness 
(density of coloring matter) of the glass, namely / = 
he~^'^y a simple method was devised, h represents 
the original intensity of light of a certain wave-length 
and /, its intensity after traversing a thickness d of 
the colored glass and e, the extinction coefficient. 
By considering the reflection from the two surfaces 
of the glass a relation was deduced in the form of 



92 COLOR AND ITS APPLICATIONS. 

Log T = log 0.92 -\-kd (where T is the transmission, d 
the thickness and k a constant) which is sufficiently 
accurate for ordinary purposes in the spectrophoto- 
metric analysis of the transmission characteristics 
of colored glasses and other media. The term *log 
0.92' can be eliminated by obtaining the transmis- 
sion of the colored glass in terms of a clear glass 
if so desired. This method necessitates an analysis 
of only one thickness, for, on plotting these data on 
logarithmic paper, as shown in Fig. 45, the data for 
various other thicknesses (even thicker than the 
sample) are readily obtained. Proof of the accuracy 
of this method is shown by the fact that the circles 
which represent data obtained on the same sample 
of glass at five different thicknesses lie close to the 
straight lines indicated by the mathematical relation 
expressed above. See Chapter XVII. 

The spectrophotometric examination of colored 
media is valuable inasmuch as the eye not being 
analytic, other methods fail to reveal the true spectral 
character of the light emitted by the colored medium. 
This was demonstrated by the three yellows in Fig. 
17 which appeared of the same hue (and practically 
of the same saturation), but differed greatly in spectral 
character. 

A spectrophotometer is an elaborate and expensive 
instrument, therefore where the need for such an 
instrument is not great enough to warrant its pur- 
chase, an ordinary spectrometer with modifications 
can be made to serve the purpose. There are various 
ways of converting ordinary spectrometers into in- 
struments satisfactory for spectrophotometric work. 
A double bilateral slit and a combination prism for 
transmitting and reflecting respectively two juxta- 
posed beams of light from the different sources into 



ANALYSIS OF COLOR 



93 



the collimator is a ready means of converting a 
spectrometer into a spectrophotometer. However, the 
comparison field which consists of narrow lines juxta- 
posed endwise is not very 



sr 



— -^zi— q_s 



^ 



Fig. 46. — Abney*s spectrophotometric 
attachment for a spectrometer. 



satisfactory. Abney used 
the scheme illustrated in 
Fig. 46 in his early studies 
in color. Two slits, SS, 
were placed in a plane at 
right angles to the col- 
limator. One slit was be- 
low the other, so that their respective images could be 
reflected toward the coUimating lens by the two right- 
angled prisms which were placed one below the other. 
This arrangement no doubt yielded a photometric 
field which was not divided by an invisible line, as is 
desirable for high sensibility. ^^ 



I 




HZ^ 



-3 



a- b 



Fig. 47. — Ives* spectrophotometric attachment for a spectrometer. 

A more satisfactory method is illustrated in Fig. 
47. This arrangement, used by Ives,'' was designed 
chiefly to avoid the errors due to instruments having 
two collimators becoming asymmetrical and also to 
avoid errors due to scattered light. At 1 in Fig. 



94 



COLOR AND ITS APPLICATIONS 



47 is placed a combination of two right-angle prisms 
cemented together. The face b is entirely silvered 
and face a silvered halfway up. A lens at 2 forms 
an image in the field of the telescope tube at 3 which 
is observed by means of an ocular lens at 4. The two 
light sources are placed at 6 and 7 respectively. A 
large monochromatic field is obtained which is equally 
affected by scattered light if any is present. Further- 
more colored glasses can be judiciously used to elimi- 
nate scattered light if necessary. 

A further improvement of the foregoing attach- 
ment, which was added by Nutting, ^ is illustrated in 
Fig. 48. The attachment consists of two reflecting 

prisms. Pi and P2, two 
Nicol prisms, Ni and N2, 
and a lens arranged as 
shown in the figure. The 
whole can be attached to 
the slit of any spectrometer. 
The essential factor is that 
a real image of the photo- 
metric field (the common 
surface of the two reflect- 
ing prisms) is thrown on 
the slit by an achromatic 
lens and is thus brought into the plane of the slit. 
The two beams of light to be compared, one passing 
through a portion of the photometric surface and the 
other being reflected by the other portion which is 
silvered, are brought to a brightness balance for 
any wave-length by rotating the Nicol prism M. 
High sensibility is claimed by Nutting for an instru- 
ment of this type. 

27. The Monochromatic Colorimeter. — Colorim- 



1 Ma 




Fig. 48. — Nutting's spectrophotomet- 
ric attachment for a spectrometer. 



eters vary in design, depending upon the data to be 



ANALYSIS OF COLOR 95 

obtained. In some industrial processes tintometers 
are employed which determine the color of substances 
in terms of arbitrary standards. Such instruments are 
colorimeters, but give no quantitative analyses of the 
colors. Their purpose is largely to keep the product 
within certain limits as to color, but they perhaps serve 
the purpose in many of these cases as satisfactorily as 
a more complex instrumenti Of the instruments that 
analyze colors into the three terms *hue,' * satura- 
tion' and * brightness,' the Nutting^ colorimeter, being 
of the latest type, has been chosen for description. The 
optical system of this instrument, which has been 
called a monochromatic colorimeter, is shown in Fig. 
49. Light entering the slit of collimator, Aj which 
is movable, traverses the 
prism and is dispersed by 
prism P into its spectral 
components, thus furnish- 
ing the measurement of 
hue of the unknown light 
which enters through the 
slit of collimator C and is 

reflected by a portion of ''^^•'^•"^^^^^fS?'''''^''^''^ 
the diagonal surface in L, 

which is a Lummer-Brodhun photometer cube. White 
light enters the slit of collimator B and is reflected by 
the prism face and joins a portion of the beam from 
A. The eye placed at the ocular slit in D sees an 
ordinary photometric field, the two parts of which 
can be matched in hue, saturation, and brightness. 
The hue is matched by varying the angular position 
of A and the saturation by varying the amount of 
white light added. The amounts of light entering 
the slits can be varied by changing the slit widths, 
by rotating sectors, or by rotating one of a pair of 




96 



COLOR AND ITS APPLICATIONS 



Nicol prisms placed just inside the slits. In analyz- 
ing a purple, for which no spectral match in hue 
exists, a spectral color is mixed with the unknown, 
the remaining procedure being obvious. The later 
instruments have been altered somewhat in con- 
struction, but the principle remains the same. The 
accuracy with which the dominant hue is obtainable 
is claimed to be about .001 to .002jLt except in the 
extreme regions of the spectrum, for very unsaturated 
colors and dark shades. Data obtained by Nutting « 
are given in Table V. 



TABLE V 



Materials 



Hue 


Per cent 




white 


0.571m 


48 


.586 


56 


.680 


9 


.697 


65 


.695 


70 


.680 


45 


.580 


28 


.472 


90 


.511 


56 


.582 


65 


.597 


70 


.575 


60 


.583 


61 


.591 


64 



Reflection 
coefficient 



Sulphur 

Cork 

Dandelion 

Tobacco leaf (medium) 

Chocolate 

Butter, light 

Butter, dark 

Navy blue (U.S.) 

Paris green 

Manila paper 

Copper 

Brass, light 

Brass, dark 

Gold, medium 



0.80 



.14 
.05 

.64 

.019 

.386 

.57 

.23 

.32 

.26 

.21 



Data obtained by Abney ^ in the analysis of the 
color of glasses and pigments are presented in Table 
VI. 

In Table VII are given some data on the color of 
illuminants obtained by L. A. Jones ^ with the mono- 
chromatic colorimeter. 



ANALYSIS OF COLOR 



97 



TABLE VI 



Glasses 



Ruby 

Canary 

Bottle-green 

Signal-green 

Cobalt 

Pigments 

Vermillion 

Emerald-green 

French ultramarine blue . 

Brown paper 

Orange 

Chrome-yellow 

Blue-green 

Eosine dye 

Cobalt-blue 



Hue 



Dominant 
hue 



0.622/z 
.686 
.661 
.4926 
.610 
.4676 



Saturation 



Per cent 
white 



26 
31 
32 
61 
42 



Brightness 



Transmis- 
sion coef- 
ficient 



0.131 
.820 
.106 
.069 
.194 
.038 



Dominant 
hue 



Per cent 
white 



0.610/x 
.622 
.472 
.694 
.6916 
.6836 
.6006 
.640 



2.6 
69 
61 
60 

4 
26 
42.6 
72 
66.6 



Reflection 
coefficient 



0.148 
.227 
.044 
.26 
.626 
.777 
.148 
.447 
.146 



TABLE Vn 



Source 



Sunlight 

Average clear sky 

Standard candle 

Hefner lamp 

Pentane lamp 

Tungsten glow lamp, 1.26 w. p. c 

Carbon glow lamp, 3.8 w. p. c 

Nernst glower, 1.5 w. p. c 

Nitrogen-filled tungsten lamp, 1.00 w. p. m. h. c. 
Nitrogen-filled tungsten lamp, 0.6 w. p. m. h. c. 
Nitrogen-filled tungsten lamp, 0.36 w. p. m. h. c. 

Mercury vapor arc 

Heliiun tube 

Neon tube 

Crater of carbon arc at 1.8 amperes 

Crater of carbon arc at 3.2 amperes 

Crater of carbon arc at 6.0 amperes 

Acetylene flame (flat) 




98 COLOR AND ITS APPLICATIONS 

In colorimetric work a standard white light is 
necessary. Jones used noon sunlight, which he found 
to be constant in color from 9 A.M. to 3 P.M., the obser- 
vations extending over several weeks. This light was 
reflected into the instrument from a magnesium car- 
bonate block. 

Many interesting studies in color-mixture can 
be made with such a colorimeter. An example is 
found in the work of L. A. Jones ^ in the analysis of 
mixtures of two component colors. Filters were 
chosen in several cases practically complementary 
in color. These filters were in the form of sectors 
of a circle and of equal angular extent. An opaque 
sector equal in size to one of the filters was varied 
in position over the sectors, so that they could be left 
open in any desired proportions. Lights passing 
through these filters were mixed in a complete range 
of ratios and the resultant mixtures were examined 
by means of a monochromatic colorimeter for hue 
and saturation or per cent white. For example, we 
will choose one of the pairs of filters, a red and blue- 
green of dominant hues 0.624jLt and 0.497/x respec- 
tively. The saturation or purity of the colors are 
indicated by the per cent of white light (noon sun- 
light) that each transmitted, these being for the red 
and blue-green filters respectively 3.3% and 28%. 
The transmission coefiicients of the two filters were 
respectively 24% and 16%. The data obtained in 
analyzing various mixtures of the two colored lights 
are shown in Fig. 50. It is seen that practically only 
two hues are obtained in a complete range of mix- 
tures and these are the dominant hues of the re- 
spective colored lights. The dominant hue of the 
mixtures changes abruptly from the hue of one of 
the colored lights to that of the other at the point 



ANALYSIS OF COLOR 



99 



near where the mixture contains the maximum 
amount of white light, or in other words where the 
two lights are nearest to being complementary. The 
per cent white reaches a maximum of 95% (indi- 
cating that the colored lights are here practically com- 
plementary) when the blue-green filter was open 
about 62% and the red filter about 38%. A con- 
clusion, among others drawn by Jones from this in- 

100 
90 



80 
70 
60 
50 
40 
50 
20 
10 













/ 


'-N, 


















/ 


N 


hv 














/ 


/ 


-"«i^ 


\ 














/ 






— N 


\ 










y 










\ 






> 


y\ 












\ 


V 




/ 
















\, 




/ 


















y 


/ 










I. 








o^ 










■ 











0.640 
0620 ^ 
0.600 ^ 

0.580 5 

z: 

0.560 E 
o 

0.540 .. 

0.520 

0500 



10 20 30 40 50 60 70 80 90 100 
COMPOSITION OF mixture: IN PER CENT. OF BLU£-GREEN FILTER 

Fig. 60. — Analysis of two-component color-mixtures. 

vestigation, is that it is not possible with two filters 
that are complementary or nearly so to produce mix- 
tures that show appreciable color of more than two 
dominant hues, these hues being the dominant hues 
of the two components of the mixture. He points 
out other possibilities for this colorimeter in investiga- 
tions in color-mixture. 

The author has used an arrangement diagram- 
matically shown in Fig. 51 for the study of various 
problems, chiefly that of the influence of saturation 
of color in heterochromatic photometry. This ar- 
rangement has all the essentials of a colorimeter for 



100 



COLOR AND ITS APPLICATIONS 



analyzing colors into hue, saturation, and brightness. 
Light from a source L emitting light of a continuous 
spectrum enters the collimator of a Hilger spectro- 
scope and is dispersed by the prism. A standard 
white light illuminates the non-selective ground opal 
glass, O, an image of which is reflected into the 
objective telescope from the prism face as shown. 
A white sectored disk, Z>, which is smoked with 
magnesium oxide by holding near a burning mag- 
nesium ribbon, is placed so that it bisects the field. 






5>>..(|: 



/ 



/ 



/ 



/PI 



u 



f- 



D 



D^ 




Fig. 51. — A simple method of converting a spectrometer into a combined mono- 
chromatic colorimeter, direct comparison photometer, flicker photometer, 
and spectrophotometer. 

F^ vertically. If the edges of the sectors are beveled 
and well sharpened, the dividing line can be made to 
disappear almost completely. The light from the 
unknown, JJy is reflected from the disk. By varying 
the intensity of the various lights the desired meas- 
urements can be made. The hue is determined by 
the position of the wave-length drum; the amount of 
white light can be measured by comparing with a 
standard at C/ or by a previous calibration. The 
brightness can be measured either by the direct 
comparison or the flicker method of photometry (# 55). 
The sectored disk provided with a motor drive is in 
reality a Whitman-disk flicker photometer. As al- 
ready stated the arrangement was originally devised 
for another investigation; however, it readily serves 



ANALYSIS OF COLOR 101 

the requirements of a monochromatic colorimeter. 
Transparent colored media can be illuminated by a 
standard white light placed at U. Likewise opaque 
colored media can be placed on the disk D and 
illuminated by a white light. 

28. The Tri-color Method. — It is well known 
that any color can be matched in hue by mixing the 
three primary colors, red, green, and blue, in proper 
proportions. The Young-Helmholtz theory of color 
vision is largely based on this experimental fact (#47). 
Koenig found, by a rather complex method, the rela- 
tive amounts of the three primary color sensations 
aroused by the various spectral colors and determined 



KO^^x 




Fig. 52. — Illustrating the principle of the Maxwell ' color box.' 

the so-called sensation curves of the eye (Fig. 54). 
Maxwell was one of the first to obtain quantitative 
data in matching colors by a mixture of three primary 
spectral colors. His apparatus, known as the * color 
box,' though somewhat more complex, involved the 
fundamental principle shown in Fig. 52, and is based 
upon the fact that an optical path is * reversible.' 
For example if a spectrum is formed at A by means 
of a collimator and a prism by light entering the slit, 
S, of the collimator and traversing a prism, we can 
obtain a patch of light of any color by placing slits, 
/?, G, and 5, in the spectrum and combining the 
light from these on a distant screen. Conversely, 
if the latter slits be illuminated with white light on 
looking into the collimator slit, S, the prism face will 
appear of a color which is the result of the mixture 
of the colors of the slits which, in the first case, were 



102 COLOR AND ITS APPLICATIONS 

combined by means of a lens into one colored patch 
on the distant screen. Hence, instead of forming a 
spectrum at Ay and producing colored light by mix- 
tures of jR, G, and 5, by slits placed at these points 
and combining the three colored lights, Maxwell 
adopted the reverse process. He illuminated the 
three slits by sunlight reflected from a white diffus- 
ing surface placed in front of them, and on looking 
into the slit, S, he saw the prism face appear in colors 
corresponding to the positions and proportions of /?, 
G, and B. This composite color he compared with 
the original white light or any colored light. The 
Maxwell color box was actually constructed in a 
different manner, but the principles involved are the 
same as indicated above. By means of this instru- 
ment he obtained many color equations of the form 
xR + yG + 25 = C. By a similar method Koenig 
obtained data which resulted in the production of the 
so-called sensation curves of the eye. Abney has 
employed the method in a great deal of work in color 
analysis, including the study of color vision, the analy- 
sis of pigment colors, and of the color of illuminants. 
A colorimeter based upon the tri-color method 
of analysis was developed by F. E. Ives.^^ Instead 
of spectral colors, red, green, and blue colored filters 
are employed in this instrument, which is illustrated 
in Fig. 53. By means of this instrument colors are 
analyzed in terms of the colors of the filters /?, G, 
and B, These can be reduced to sensation values 
as shown later. Z) is a variable slit which is illumi- 
nated by light of the color to be analyzed and A is 
an optical mixing wheel consisting of twelve convex 
lenses arranged to rotate. By means of this wheel 
the various amounts of the red, green, and blue com- 
ponents are mixed to match the light from D. F 



ANALYSIS OF COLOR 



103 



is the field lens and C a prism or small angle which 
divides the photometric field by a sharp line in the 
middle. H is the eyepiece, 7 is a hinged front 
carrying the objective lens, K^ and prismatic lens 
L. These are unnecessary for some work and can 
be replaced by a non-selective ground opal glass. 
The procedure in making observations with this 




^\^ ^m^/y/yy////////////////^^^^^^ 




^//<r/^//////^//^//////^^^^^ 



Fig. 53. — The F. E. Ives colorimeter. 

instrument is obvious. If the colorimeter readings 
which are obtained from the position of the levers 
which control the slit widths of /?, G, and 5, be 
reduced to sensation values they become much more 
valuable. H. E. Ives ^^ has done this in analyzing 
the color of illuminants, by using the sensation curves 
obtained by Kbenig and modified by Exner. These 
are shown in Fig. 54. They are based upon experi- 
mental data which has afiforded strong confirmation 
of the Young-Helmholtz theory of color vision which 
assumes three fundamental color sensations are 
responsible by different degrees of excitation for 



104 



COLOR AND ITS APPLICATIONS 



the perception of all colors (#47). It is noted that 
each of the three supposed primary color sensations 























































r 


\ 


Vue 
































z: 
o 

r- 






/ 






\ 










^-. 


^Oreen 


















^ 






/ 






\ 








/' 




^ 


s. 




















s/5 




/ 








\ 






/ 


Red^-^ 


^ 


S. 


















UJ 

> 




/ 










\ 




1 


/ 


/ 






V 


\ 
















?, 




/ 










\ 


y 




/ 








\ 




\ 














LlJ 

a: 


/ 












\ 


/ 


1 












\ 




\ 














/ 












/ 


\\ 


/ 












\ 


V 


\ 


N 












/ 




^^ 






>^ 


— ■ 




"'"»«>, 

















"^ 













0.59 042 046 045 Q5I 0.54 0.57 0.00 0.65 0.66 0.68 0.72 
Fig. 64. — Koenig's sensation curves. 

is not excited by a limited portion of the spectrum. 
In fact, spectral rays in general are supposed to 




Fig. 55. — Tri-color colorimeter measurements. 

excite the sensations in relatively different degrees, 
depending upon the wave-length. After a prelimi- 
nary investigation Ives concludes that these curves 
are a much nearer approach to the truth than those 
obtained by Abney. In reducing the colorimeter 



ANALYSIS OF* COLOR 



105 



readings to sensation values it was necessary to 
obtain the red, green, and blue sensation values of 
the colorimeter screens. Spectrophotometric analy- 
sis of the screens combined with the data in Fig. 
54 yield the primary sensation values of the screens 
which are obtained in relative values by integrating 
the areas under the sensation curves for the three 
screens and reducing the colorimeter readings accord- 
ingly. Each of the three colorimeter readings repre- 
sents a mixture of the three primary sensations, 
depending upon the primary sensation values of the 
colorimeter screens. The procedure is simple but 
more details, if desired, can be obtained from the 
original paper. The primary sensation values of 
various illuminants compared with average daylight 
as determined by Ives are found in Table VIII, some 
of which are plotted in Fig. 55. The data represent 



TABLE Vm 
Color of niuminants by Tri-chromatic Colorimeter (See Fig. 56) 



Source 



Sensation values 



Red Green Blue 



1. Black body 5000° abs 

2. Blue sky (S) 

Blue sky (C) 

3. Overcast sky 

4. Afternoon sun 

5. Hefner lamp 

6. Carbon incandescent lamp, 3.1 w. p. m. h. c. . . 

7. Acetylene 

8. Tungsten incandescent lamp, 1.25 w. p. m. h. c. 

9. Nemst 

10. Welsbach, I % cerimn 

11. Welsbach, | % ceriiun 

12. Welsbach, 1^ % cerium 

13. D. C. Arc 

14. Mercury arc 

15. Yellow Flame arc 

16. Moore carbon-dioxide tube 



33.3 
26.8 
32.0 
34.6 
37.7 
54.3 
51.1 
48.6 
48.3 
49.2 
42.5 
45.4 
47.2 
41.0 
29.0 
62.0 
31.3 



33.3 
27.2 
32.0 
33.9 
37.3 
39.5 
40.5 
40.8 
40.8 
40.7 
40.8 
42.0 
41.8 
36.3 
30.3 
37.5 
31.0 



33.3 
46.0 
35.8 
31.6 
25.0 
6.2 
6.4 
10.6 
10.9 
11.1 
16.7 
12.6 
11.0 
22.7 
40.7 
10.6 
37.7 



106 



COLOR AND ITS APPLICATIONS 



the means of the values determined by two methods, 
namely colorimeter readings and likewise spectro- 
photometric data reduced to sensation values. (The 
primary color sensation values of the spectral colors 
and principal lines of the cadmium and mercury 
spectra are plotted in Fig. 31.) The dotted line 
represents the color of a black body (or an incandes- 
cent solid emitting radiation non-selectively) for tem- 
peratures between 3000 and 7000 degrees absolute 
(C). Most of the artificial illuminants lie along this 
curve. Those radiating selectively in the visible 
spectrum, such as the yellow flame arc and Wels- 
bach mantle, do not lie upon it. Ives concludes that 
the spectral distribution of energy in noon sunlight 
which reaches the earth's surface is quite similar to 
that of the black body at 5000 degrees absolute (C) 
as computed from radiation laws (#6). 

Another instrument for tri-color analysis which 
is extremely simple is illustrated in Fig. 56. This 

Green method, which has 

Red __• Qj^^ been applied by many 
in various color investi- 




Fig. 56. — Arrangement for using color 
filters before a photometer eyepiece. 



gations, has been used 
by Bloch.i2 ^ ^jgj^ ^q^- 

taining four circular 
apertures, three being 
respectively covered 
by red, green, and blue screens, is pivoted so 
that the various screens can be brought before 
the ocular aperture in a photometer head. Pho- 
tometric balances are made while viewing the field 
through the various filters separately and the re- 
sults are plotted on rectangular coordinates, the ratio 
of red to green intensities being plotted against the 
ratio of blue to green intensities. Bloch presents 



ANALYSIS OF COLOR 107 

plats containing his color analysis of many illuminants 
and the spectrophotometric analyses of the filters 
are also shown. Such results can hardly be consid- 
ered more than approximately comparative and of 
limited usefulness. In general, data concerning the 
color of illuminants or of colored media obtained 
by the tri-color method of analysis are limited in 
usefulness, owing to the fact that the method is 
not sufficiently analytical. The usefulness of such 
a method is broader than the tintometer with its 
arbitrary standards of color, but the spectropho- 
tometer and monochromatic colorimeter as a rule yield 
more useful data, the former being quite analytical 
for spectral examination and the latter rendering data 
in terms of the specific qualities of a color, namely, 
hue, saturation, and brightness. 

29. Other Methods of Color Analysis. — Many 
instruments have been devised for color analysis 
based on principles differing from the foregoing. 
A number of colorimeters employing colored solu- 
tions have been used, the measurements usually 
being made in terms of the depth of liquids of cer- 
tain concentrations. Purple and green solutions have 
been used by Fabry for eliminating color difference 
in photometry. In a sense such a procedure is a 
colorimetric method if it is desired to use it as such. 
The Kirchoff-Bunsen and Stammer colorimeters em- 
ploy colored solutions for the measurement of color. 

Leo Axons ^^ has devised a colorimeter based 
upon the rotation of the plane of polarization by 
quartz plates (#11) which have been cut perpendicu- 
larly to their crystallographic axes. This instrument 
is illustrated in /, Fig. 57. White light from a dif- 
fusely reflecting porcelain disk is reflected into the 
instrument through a circular hole, 5, and is rendered 



108 



COLOR AND ITS APPLICATIONS 



plane-polarized by the Nicol prism, P. A quartz plate 
at Q rotates the plane of polarization of various 
rays through various angles depending upon the 
wave-length. The beam then passes through another 
Nicol prism, A^ thence through the central portion 
of the Lummer-Brodhun photometer cube, W, and 
to the eye beyond R, The eye sees a circular patch 
of light of a certain color depending upon the thick- 
ness of the quartz plate and the relative angular posi- 
tions of the Nicol prisms. This colored patch is 



\/K7/I7m 



B lQ /t7 qQ fjry ws g ) 



U B' 



0ZZ7J 



jy 



Fig. 57. — Arons colorimeter. 



matched in color with the light entering the side 
tube, N. The latter beam is controlled in intensity 
by the two Nicol prisms. Pi and P2, and is reflected 
by the totally reflecting prism, Z>, to the photometer 
cube, which in turn reflects the light to the eye in 
a beam concentric with the first beam. The colored 
media are placed in front of tube Ny and are pref- 
erably illuminated by the same source that illumi- 
nates the porcelain disk in front of B. Mixed colors 
are obtained, the Nicol, A^ subtracting certain rays 
depending upon its angular position leaving the 
remaining light colored instead of white. Six quartz 
plates are provided, of thicknesses 0.25, 0.5, 1.0, 2, 
4, and 8 millimeters respectively, which are mounted 
in brass plates. These plates have two identical 



ANALYSIS OF COLOR 109 

holes, one covered with the quartz plate, the other 
unobstructed. These are arranged to slide in or 
out of the instrument at or near Q, By sliding any 
of the brass plates to the side any number of quartz 
plates can be arranged one after another and thus 
the total thickness of quartz in the path of the beam 
from B can be adjusted in steps of 0.25 mm. to a 
total thickness of 15.75 mm. A still greater variety 
of colors can be obtained by using two sets of Nicol 
prisms and quartz plates in series. Therefore the 
tube in / can be removed at the plane xx^ and tube 
// connected at the end yy. B' takes the place of 
B and lens V the place of L, Any thickness of 
quartz plates at Q' can be inserted; however, only a 
single plate is employed by Arons, this one being 
3.75 mm. in thickness. In case transparent colored 
media are to be examined, a white diffusely reflect- 
ing porcelain disk similar to the other one is used in 
front of the tube N, The two disks should receive 
the same intensity of illumination from the same 
source. If opaque colors are to be examined for 
reflection, these are placed on the porcelain disk, 
and the observer sees an outer ring through R of the 
color of the unknown. This is matched in color and 
brightness by adjusting the thickness of quartz and 
the angular position of the Nicol prisms until the 
inner circle appears of the same color and brightness 
as the outer ring. The measurements are recorded 
in terms of the thickness of quartz, the angle between 
A and P and the angle between Pi and P^. and also 
between P' and P if the tube II is in use. 

30. Templates. — Much of the early investigation 
in color was done with the rotating disks (Fig. 23) 
and it is quite natural that modifications of these 
would be made. Abney devised an ingenious method 



110 



COLOR AND ITS APPLICATIONS 




Fig. 58. — Abney's 
template for car- 
mine. 



for showing the effect upon the color of the integral 
light of various spectral energy distributions and also 
of showing that a certain determined spectrophoto- 
metric curve was in reality the analysis of an integral 
color. On determining the relative amounts of light 
of various wave-lengths reflected by a pigment these, 
instead of being plotted on rectangular coordinates 
as shown by the dotted lines in Fig. 12, were plotted 
in a speciar manner on a disk. Along 
a portion, VRy of a radius of the circle 
in Fig. 58, a wave-length scale is laid 
off. The relative amounts of light of 
different wave-lengths reflected from 
the pigment as determined by means 
of a spectrophotometer are laid off on 
circumferences of circles concentric 
with the center of the disk starting at a 
certain point of VR corresponding to the wave-length. 
The cardboard is now cut out along the boundary line, 
the template in Fig. 58 being Abney's template for car- 
mine. If this disk be carefully adjusted in the plane 
of a spectrum formed in space so that various wave- 
lengths along VR coincide with corresponding wave- 
lengths in the spectrum and the disk be rotated, on 
combining the colored rays passing through the 
rotating aperture upon a white screen by means of a 
lens the color of the integral light reflected from car- 
mine is seen. This patch will be exactly like the 
original color in appearance providing the optical 
parts of the instruments are non-selective and the 
same light is used in producing the spectrum as 
was used in illuminating the pigment when the 
spectrophotometric observations were made. Of 
course the irrational dispersion of the prism must be 
properly allowed for and the spectrum must be narrow. 



ANALYSIS OF COLOR 



111 



Instead of rotating the template before an actual 
spectrum Abney used the principle adopted by Max- 
well in his * color box' (Fig. 52), thus rotating the disk 
before a long narrow slit illuminated by the total 
light from the illuminant. The integral color was 
viewed through the eyepiece of the spectrometer. 
Abney made a number of these templates represent- 
ing pigments, illuminants, and the luminosity curve 
of the eye. 




Elevation 



Enp View 



Fig. 59. — Adaptation of Abney's scheme for the spectroscopic synthesis of color. 

Recently Ives and Brady ^^ applied Abney's prin- 
ciple to the alteration of the light from a 4 w.p.m.h.c. 
carbon lamp to that of * average daylight' and also 
to that from the blue sky. A Hilger constant-devia- 
tion spectrometer was used, as shown in Fig. 59. 
The regular camera attachment B was placed in the 
position ordinarily occupied by the collimator, the 
latter being placed in the position of the objective 
telescope. The slit at S is long and narrow and is 
illuminated by light from the carbon incandescent 
lamp reflected from a white surface at F, the prin- 
ciple being the same as just presented in the 
description of Abney's work on templates. These 
templates were computed on the assumption that the 



112 COLOR AND ITS APPLICATIONS 

relative spectral energy distribution in the spectrum 
of the carbon incandescent lamp operating at 4 watts 
per mean horizontal candle is that derived from the 
Wien equation (equation 2, #6) for a black body at 
a temperature of 2080 deg. absolute (C), and that of 
white light corresponding to a temperature of 5000 
deg. absolute. The templates for converting the 
carbon light into blue sky light were made from 
relative spectrophotometric measurements. The disk 
in position is shown in the three views of the appa- 
ratus taken from the work cited above. The advan- 
tage of using the templates before a slit illuminated 
by white light is that a much greater amount of 
light is available than in the case of using it before 
a spectrum and recombining the transmitted light 
by means of a lens. A comparison field can be 
arranged by reflecting the light L into the instrument 
as shown. Abney cut a template corresponding to 
the luminosity curve of the eye which is of interest, 
but owing to the work of various modern investi- 
gators this has been more accurately established. 
The template scheme can be applied by using disks 
in which openings are cut corresponding to the lumi- 
nosity curve of the eye and replacing the surface 
at F before the objective slit by a straight incandes- 
cent filament; thus the transmissions of absorbing 
media can be determined by pure energy measure- 
ments. 

31. The Nutting Reflectometer. — In the study 
of color it is sometimes desirable to ascertain the 
reflection coefficients of colored media. This can 
be done if the object is diffusely reflecting by means 
of an ordinary brightness photometer, although the 
uncertainties of color photometry will be present in any 
case. However, Nutting ^^ has devised a simple instru- 



ANALYSIS OF COLOR 



113 



ment shown in Fig. 60 that is very useful for deter- 
mining the reflection coefficients of any colored media 
for light incident from all possible directions simul- 
taneously. Two crown glass 
prisms of 21 deg. angle are 
fastened over the two 
apertures in the end of a 
Koenig-Martens polariza- 
tion photometer and the lat- 
ter is inserted into a metal 
ring which is nickel-plated 
and polished inside. The 
light enters the apertures of 
the instrument along the 
dotted lines shown and is 
divided into two plane-polar- 
ized beams by a Wollaston 
prism. These beams can 
be balanced in intensity by 
rotating the Nicol prism. 




Fig. 60. 



The 



The Nutting reflectom- 
eter. 



surface whose re- 
flection coefficient is desired is placed on one 
side of the ring completely covering it and this is 
illuminated by a non-selective ground opal glass 
on the other side of the ring. The instrument is 
placed upon a wooden frame for convenience. The 
light is reflected back and forth between two planes 
of infinite' extent made practically so by the polished 
ring. Simple theory shows that the ratio of the 
brightness of the unknown to that of the ground opal 
glass is a direct measure of the reflection coeffi- 
cient of the former for the character of the illumina- 
tion it receives. Certain precautions must be taken 
into consideration as explained by Nutting. 

32. Methods of Altering Brightness of Colors 
Non-selectively, — It is often desirable to alter the 



114 COLOR AND ITS APPLICATIONS 

brightness of colored lights without altering them 
spectrally. A simple means is found in varying the 
distance of the light source and computing the rela- 
tive intensities from the * inverse square law.' How- 
ever, sometimes this is inconvenient. Sectored disks 
are often resorted to with satisfactory results. These 
are now being used in photometry to a great extent, 
the variable sectored disk devised by Hyde (Fig. 44) 
being especially convenient and reliable for spectro- 
photometry. The Brodhun variable sector is another 
device very often applicable. In this instrument a 
beam of light is rotated and is controlled in intensity 
by a variable stationary sector. Plate glass varied 
in its angular position with respect to the axis of the 
beam affords a means of obtaining a slight range of 
brightnesses, although non-selective glass is rarely 
found. Wire mesh and grids thoroughly blackened 
are satisfactory in some problems. Neutral tint 
wedges have been used, but it is difficult to obtain 
strictly non-selective smoke glass. Ives and Luck- 
iesh 1^ studied the transmission characteristics of 
half-tone gratings (black lines on clear glass) and 
found them to be satisfactory if properly used. Pho- 
tographic screens are found to serve some purposes, 
but they must always be calibrated in position owing 
to their tendency to diffusely reflect light. These 
are a few methods which have proved helpful in the 
proper place. 

REFERENCES 

1. Bul. Bur. Stds. 1906, 2, p. 317. 

2. Astrophys. Jour. 1912, 25, p. 239. 

3. Trans. L E. S. 1914, p. 853. 

4. Phys. Rev. 1910, 30, p. 446. 

5. Bul. Bur. Stds. 7, p. 234. 

6. Bul. Bur. Stds. 1913, 9, No. 187. 



ANALYSIS OF COLOR 115 

7. Color Mixture and Measurement, p. 165. 

8. Trans. I. E. S. 1914, 9, p. 687. 

9. Phys. Rev. N. S. 1914, 4, p. 454. 

10. Jour. Franklin Inst. July, Dec. 1907. 

11. Trans. I. E. S. 1910, p. 189. 

12. Electrotech. Zeit. 1913, 46, p. 1306, 

13. L'Industrie Elec. July 25, 1911. 

14. Jour. Franklin Inst., 178, p. 89. 

15. Trans. I. E. S. 1912, 7, p. 412. 

16. Phys. Rev. 1911, 32, p. 522. 



CHAPTER VI 

COLOR AND VISION 

33. The Eye. — Color vision is not essential, 
because achromatic vision serves the totally color- 
blind person well. However, the ability to perceive 
colors extends the usefulness of the sense of 
sight very much. It not only adds greatly to our 
pleasure but is utilized in many ways. The eye 
can be considered optically as a rather simple instru- 
ment, as indicated by the photograph of the middle 
vertical section of a human eye shown in Fig. 61. 

It is seen that the refracting 
media consist of the cornea, 
aqueous humor, lens, and vitre- 
ous humor. The retina, which 
consists of the optic nerve 
spread out over the interior 
of the eyeball, is a very thin 
membrane, and can be seen 
in the illustration partially de- 

'1ke^;;»1;:\f:^"^'°°°' tached from the wall. The 

radii of curvature, thickness, 
and refractive index of the various eye media as 
determined by Helmholtz ^ are given in Table IX. 
The normal eye, while being a wonderfully adapt- 
able instrument, is not free from errors, owing 
to the fact that it is optically quite simple. The 
chief error of interest here is its lack of achromatism. 
If an image of an object illuminated by light having 
a continuous spectrum be produced by a simple lens 

116 




COLOR AND VISION 



117 



TABLE IX 
Optical Constants of the Eye 



Index of refraction of the humors and cornea 

Index of refraction of the crystalline lens 

Effective index of refraction of lens surrounded by humors 

Radius of outer stirface of cornea 

Radius of first lens surface 

Radius of second lens surface 

Thickness of cornea 

Thickness of crystalline lens 

Distance of first lens surface from cornea 

Distance of second lens surface from cornea 



Distant 



1.3365 
1.4371 
1.0753 
7.8 mm. 

10.0 
6.0 
0.4 
3.6 

.3.6 
7.2 



Near 
vision 

(15 cm.) 



7.8 mm. 

6.0 

5.6 

0.4 

4.0 

3.2 

7.2 



it will be found to have a red, blue, or purple fringe. 
This is readily understood from Fig. 62, which repre- 
sents a schematic eye in which only the simple lens 
is considered. Owing to the difference in the refrac- 
tive index of a medium for rays of different wave- 
length, such a result as is exaggerated in Fig. 62 will 
obtain. The refractive index being greater for rays 
of shorter wave-length, the blue rays will be deviated 
or refracted more than the yellow rays, and the 
latter more than the red rays. Naturally the eye 
focuses for the brightest rays, which in ordinary 
light are the yellow-green or yellow rays. There- 
fore, the blue and red rays will be out of focus, 
with the result that the image of the point, P, will 
be surrounded by a purple fringe. This is of im- 
portance in vision, as will be shown later. The lack 
of achromatism of the eye can be demonstrated very 
simply. On viewing, by reflected light, the concentric 
circles shown at the right of Fig. 62 held close to 
the eye they appear colored. A very striking experi- 
ment is found in focusing a line spectrum — that 
of mercury will suflBice — upon a ground glass. On 



118 COLOR AND ITS APPLICATIONS 

viewing it at a normal distance (14 inches), the yellow 
and green lines will appear sharply focused, but the 
blue and violet lines will appear hazy and quite out 
of focus. On bringing the eye closer the latter lines 
will begin to appear clearer, and finally, when the eye 
is within about six inches of them, they will still 
appear clear-cut, while it will be quite impossible to 
accommodate the eye sufficiently to focus 'the yellow 
and green lines. In other words the eye is near- 
sighted (myopic) for blue rays and far-sighted (hyper- 
opic) for red rays. On viewing a narrow continuous 
spectrum at some distance the blue end appears to 




Fig. 62. — Showing the effect of chromatic aberration in the eye. 

flare out. Another simple demonstration is found 
in viewing an illuminated slit through a dense cobalt 
glass which transmits extreme red and violet rays. 
On accommodating the eye for a point behind the slit 
a red image with a violet halo is seen. On accommo- 
dating for a point in front of the slit a violet image 
with a red halo is seen. This defect plays a promi- 
nent, though usually unnoticed, part in vision. A 
lens can be made practically achromatic by combining 
a convergent lens of crown glass with a divergent lens 
of flint glass. The former is more strongly conver- 
gent for blue than for red rays, while the latter is 
more strongly divergent for blue than for red lights. 
It is thus possible to bring the red and blue rays in 
coincidence at a focus. Inasmuch as it is only pos- 
sible to bring two rays exactly into coincidence by 




COLOR AND VISION 119 

a two-piece lens, such a lens is not truly achromatic, 
though practically so for most purposes. By com- 
bining more lenses the approach 
to true achromatism is brought 
as near as desired. A simple 
achromatic lens is illustrated in 
Fig. 63. 

The retina has been found 
to vary in its sensibility to colors. 
The central region is sometimes 
known as the yellow spot, be- 
cause it apparently absorbs the Fig.63.-A simple achromatic 

violet and blue rays to a greater 

degree than other rays. The effect of the yellow spot 
is often seen in viewing colors one after another, and 
it is quite noticeable at twilight illumination. It 
appears of somewhat irregular outline in after- 
images. Studies of the various zones of the retina 
as to their sensibility to various colors yield results 
in general similar to those shown in Fig. 64. The 
center of the fovea corresponds to the center of 
the circle. The solid line shows the boundary for 
the perception of light. The visual field for one 
eye extends outward about 90 deg. from the normal 
optical axis of the eye, inward about 60 deg., down- 
ward 70 deg., and upward 50 deg. The dashed line 
represents the extreme limits where blue can be 
perceived as such and the remaining two lines repre- 
sent respectively the limits for red and green per- 
ception. These facts must be reconciled with any 
satisfactory theory of vision. It might be noted here 
that each eye has a blind spot — the point of entrance 
of the optic nerve — which is totally insensitive to light. 
The retina, which consists of the optic nerve spread 
out, is covered with a mass of microscopic * rods' and 



120 



COLOR AND ITS APPLICATIONS 



'cones' (#48) projecting outward toward the lining of 
the eyeball which play an important part in theories 
of vision. 

34. Brightness Sensibility. — The sensibility of 
the retina to brightness differences is greatest over 
a wide range of intensities, falling off at extremely 




Co/or/ess 
Blue 



.. Red 
- Oreen 



Fig. 64. — Limits of the visual field for colored and colorless lights. 

low and extremely high brightnesses. With decreas- 
ing intensities the sensibility diminishes more rap- 
idly for rays of longer wave-length than for those of 
shorter wave-length. Kbenig and Brodhun ^ have 
done excellent work in this field as well as in many 
other fields pertaining to vision. They determined 
the least perceptible brightness increment for lights 



COLOR AND VISION 



121 



of various colors including white, for brightnesses of 
a neutral tint surface (* white') illuminated to various 
intensities from 1,000,000 meter-candles to nearly the 



^ 1-0 

UJ 

z: 

X 0.8 

is 04 

ujz: 

§ 0.2 

2: 




w 


\ 


















"A 


^ 


\ ■ 



















\ 


1 








■ 








"«^ 


V 


^ 


















X 


^ 


N 


^^ 











-4 



-3 



-2-10 1 2 5 

LOGARITHM OF ILLUMIMATIOM.! 



Fig. 65. — Brightness sensibility data. (See Table X.) 



threshold of vision, using an artificial pupil of 1 sq. 
mm. area. They started at 600 meter-candles and 
extended the illumination above and below by the 
various steps indicated in the accompanying table. 
The data for Kbenig's eye after modification by 
Nuttings are shown in Fig. 65 and Table X. Koenig 
and Brodhun did not include the increment (bB) 
in the total brightness (J5) in calculating the values 
bB/B. Nutting recomputed the data with the thresh- 
old value included. It is seen that the increment of 
brightness difference just perceptible, increases as 
the brightness decreases and more rapidly for the 
rays of longer wave-length. At high illuminations 
the minimal perceptible increment is about the same 
(1.6%) for all colors, including white. For the ordi- 
nary range of brightnesses 55/5, is constant, which 
fact is known as Fechner's law, and the constant is 
called Fechner's coefficient. 



122 



COLOR AND ITS APPLICATIONS 



TABLE X 

Data of Koenig and Brodhun on Brightness Sensibility as 
Recalculated by Nutting 



Wave- 










1 




length = 


= 0.670m 


0.605m 


0.575m 


0.505m 


0.470m 


0.430m 


Bo = 


= 0.060 


0.0056 


0.0029 


0.00017 


0.00012 


0.00012 


Meter 






SB 








Candles 






B 








200,000 




0.0425 










100,000 




0.0241 


0.0325 








50,000 


0.0210 


0.0255 


0.0260 









20,000 


0.0160 


0.0183 


0.0206 


0.0195 








10,000 


0.0156 


0.0163 


0.0179 


0.0181 







5,000 


0.0176 


0.0158 


0.0166 


0.0160 






2,000 


0.0165 


0.0180 


0.0180 


0.0176 


0.0180 




1,000 


0.0169 


0.0198 


0.0185 


0.0184 


0.0167 


0.0178 


500 


0.0202 


0.0235 


0.0180 


0.0194 


0.0184 


0.0214 


200 


0.0220 


0.0225 


0.0225 


0.0220 


0.0215 


0.0246 


100 


0.0292 


0.0278 


0.0269 


0.0244 


0.0225 


0.0246 


60 


0.0376 


0.0378 


0.0320 


0.0252 


0.0250 


0.0272 


20 


0.0445 


0.0460 


0.0385 


0.0296 


0.0320 


0.0345 


10 


0.0655 


0.0610 


0.0582 


0.0362 


0.0372 


0.0396 


5 


0.0918 


0.103 


0.0888 


0.0488 


0.0464 


0.0494 


2 


0.1710 


0.167 


0.136 


0.0656 


0.0716 


0.0600 


1 


0.258 


0.212 


0.170 


0.0804 


0.0881 


0.0740 


0.5 


0.376 


0.276 


0.208 


0.0910 


0.096 


0.0966 


0.2 




0.332 


0.268 


0.110 


0.127 


0.116 


0.10 






0.396 


0.133 


0.138 


0.137 


0.05 








0.183 


0.185 


0.154 


0.02 









0.251 


0.209 


0.223 


0.01 








0.271 


0.189 


0.249 


0.005 








0.325 


0.300 


0.312 


0.002 













0.369 



The value of the minimal perceptible increment 
depends largely upon the method of making the 
measurements. Usually the brightness of one of the 
two parts of the photometric field is varied until it 
appears just perceptibly brighter or darker than the 
comparison field. This procedure yields values of 



COLOR AND VISION 123 

the least perceptible increment comparable with the 
foregoing value. In precision photometry the accu- 
racy is often as high as 0.1 per cent; however, another 
factor enters into such procedure. The brightness 
of one part of the field is varied between certain 
limits at which it is respectively distinctly brighter 
and darker than the comparison field, and these limits 
are gradually brought nearer together until finally 
an attempt is made to estimate the middle point. 
This cannot be considered a measure of brightness 
sensibility. However, P. W. Cobb has employed a 
method which is of considerable interest here inas- 
much as he obtains values for the minimal percep- 
tible increment for white light smaller than 0.5 per 
cent. In these experiments the test field was exposed 
to the view of the observer for a brief, but constant, 
period, after which his judgment was recorded. One 
side of the field appeared either brighter or darker, 
or no difference in brightness was distinguishable. 
This procedure was repeated for a range of aspects 
of the test field varying from that in which one side 
appeared distinctly darker for a number of succes- 
sive exposures to that in which it appeared definitely 
brighter. Obviously, by progressing in small steps 
between these two limits (presenting these various 
aspects in haphazard order) there were several near 
equality where the judgment was uncertain. After 
reducing the data by a special method Cobb con- 
cludes that the minimal perceptible increment is 
much smaller than that obtained by Koenig and 
Brodhun. 

The data of Koenig and Brodhun has been ex- 
tended by Nutting by computation to the point where 
8B/B = 1; that is, to the threshold value. This 
computation is very interesting, though perhaps not 



124 COLOR AND ITS APPLICATIONS 

entirely free from criticism. Bq in Table X repre- 
sents the threshold value of brightness measured as 
a fraction of the standard high brightness. Brightness 
Bj is proportional to illumination, /, and inasmuch 
as it is a brightness that is perceived the symbol B 
is used. 

35. Hue Sensibility. — Notwithstanding the fact 
that the visible spectrum is generally considered to 
exhibit only six or seven colors, four of which, red, 
yellow, green, and blue are strikingly distinctive, 
there are theoretically present an infinite number 
of hues. The number of distinct hues that a person 
is able to distinguish depends upon the manner in 
which the experiment is conducted. Edridge-Green ^ 
states that he has 'never met with a man who could 
see more than 29 monochromatic patches in the 
spectrum.' Rayleigh,^ who is able to detect the dif- 
ference in hue of the sodium D lines (0.5890m and 
0.5896/x), could distinguish only 17 hues on Green's 
apparatus, and claims this is due to the method of 
comparing the patches. In Green's apparatus the 
principle is that of two opaque screens held over a 
spectrum and slightly separated from each other. 
One is then moved until the hue at its edge appears 
different from that at the edge of the other. With 
an apparatus employing the principle of the Maxwell 
color box Rayleigh was able to distinguish many more 
hues. By the use of spectral apparatus as high as 
128 distinctly different spectral hues have been seen. 
It is not difficult to obtain by the use of dyed media 
a series of 25 distinct spectral hues. Ridgeway,^ 
by beginning with papers dyed to represent six 
spectral hues and adding various intermediate hues, 
obtained 36 distinct hues. The data on hue sensi- 
bility vary considerably, which perhaps is due to 



COLOR AND VISION 



125 



variations in the refinement and nature of the experi- 
mental methods employed. 

Some excellent data have been obtained by Steind- 
ler 7 on hue sensibility for twelve subjects. The posi- 
tions of the maxima differed somewhat for the various 



































































































^ 
















/ 








00 








/^ 








/ 


1 






5 








f 


\ 






/ 


\ 












/ 




\ 






/ 


\ 






X 

tiJ 






/ 




\ 




) 




\ 






^ 






/ 






/ 




\ 






UJ 


f 


\ 


/ 






V. 


r 




\ 








1 


V 
















y 


\ 
























\ 

























040 0.44 



046 052 Q56 060 

^. "HML LENGTH 



Fig. 66. — Hue sensibility. 



0.64 
(Steindler's Eye.) 



observers. The hue sensibility curve for Steindler's 
eye is shown in Fig. 66 and the mean positions of 
the maxima and minima of the hue sensibility curves 
for the twelve observers and the wave-length limen 
of *just perceptible difference' are given in Table XI. 
Nutting 8 has used the mean results obtained by 
Steindler in deriving a natural scale of color. These 
mean results, including Nutting's color scale, are plotted 
in Fig. 67. The hue sensibility curve, S, was plotted 
by connecting the mean positions of the minima and 



126 



COLOR AND ITS APPLICATIONS 



TABLE XI 

Steindler's Data on Hue Sensibility 

(The mean for twelve eyes) 





Position 


Perceptible limen 


First rnaximuTn 


0.455/i 

0.534 

0.621 

0.440 

0.492 

0.581 

0.635 


0.0293m 
0.0334 


Second TnayiTniiTn ... 


Third maxitrmm 


0.0375 


First minimiiTn , 


0.0247 


Second rninitntim 


0.0136 


Third rniniTnutn 


0.0139 


Fourth TninitTiutTi 


0.0300 







maxima for the twelve observers with smooth curves. 
The limen (least perceptible difference in terms of m) 
curve, L, is plotted in the same manner. For the 



24 



20 



16 



U 





^N - 


C^^ _.-J~^: 


V V ^ IXT, 


X^ %^^'^^^T^-^ 


^■'u^^ \ ixi: 


t \ ^^A 


^t A 7kj~ 


z^^- ks/ VV .. 


/- ^^^^; 




^^ 



0.044 
0.040 -^ 
0.036 Sv 
0.032 t 
0.02.8 ^ 
0.024 5 
0.020 ^ 
0.01 & < 
0.012 g 
0.008 3 
0.00.4 
.0 



0.42 0.4& 0.50 0.54 0.5a 0.62 0J&&" 
,My, WAVE LENGTH 

Fig. 67. — Hue sensibility, limen, and color scale. 



details of the procedure adopted in obtaining the 
color curve, C, the reader is referred to the original 
paper. A difference of one unit in the color scale 
represents a difference in color that is just easily 
perceptible. It will be noted that the color curve 



COLOR AND VISION 127 

indicates there are 22 of these colors *just easily 
perceptible' within the spectral limits shown. 

36. Saturation Sensibility. — The data on the 
sensibility of the eye to changes in saturation are 
not very extensive or definite. Nutting ^ states that 
with his monochromatic colorimeter the probable 
error in the *per cent white' observations on a nearly 
spectral matte orange pigment was about ten per 
cent. L. A. Jones ^° claims an accuracy of the order 
of three per cent for the *per cent white' readings 
(Table VII) for this monochromatic colorimeter of 
improved type. The accuracy of course will vary 
with the hue, brightness, and degree of saturation 
of the colors. H. Aubert ^^ determined the smallest 
sector of color that would be just apparent on a rotat- 
ing white disk to be 2 or 3 degrees — less than one 
per cent. With black and gray disks he found that 
even smaller sectors were recognized. His experi- 
ments on the differential limen of color sensitivity 
indicated that on a black background the stimulus- 
increments for orange, blue, and red were respectively 
0.95, 1.54, and 1.67 per cent in order to produce a 
just noticeable increase in saturation. 

Geissler 12 studied the problem whether the number 
and sizes of the colored stimulus-increments corre- 
sponding to just noticeable saturation differences 
would lend themselves to a measure of saturation. 
The problem was attacked from two extremes; one 
by gradually reducing a maximally saturated pigment 
color, and the other by introducing more and more 
color into a colorless stimulus. He employed the 
rotating double color disk with the Zimmerman col- 
ored and gray papers illuminated with an artificial 
daylight devised by Ives and Luckiesh. In the first 
method he used red beginning with maximal sat- 



128 COLOR AND ITS APPLICATIONS 

uration — 360 degrees of red — for both the inner 
and outer concentric components of the double disk 
and gradually added small amounts of gray (of the 
same brightness as the red as measured with a 
flicker photometer) to the inner or smaller disk until 
it appeared just perceptibly less saturated than the 
outer or larger disk. This procedure was then re- 
versed, the outer disk being decreased in saturation 
until the change was just perceptible as compared 
with the inner disk whose saturation was kept con- 
stant. This was done for seven different degrees of 
saturation, ranging from 360° of red to 110° of red 
plus 250° of gray of the same brightness as measured 
by the flicker photometer. His results indicate that 
the stimulus-increments corresponding to just notice- 
able saturation-differences are approximately con- 
stant (about 4° of gray) at such different stages of 
saturation as 325° red plus 35° gray, 230° red plus 
130° gray, and 110° red plus 250° gray. Geissler 
states that *it seems fair to assume that the incre- 
ment-values would have remained constant at the 
intervening stages and perhaps also at a stage not 
far removed from the absolute color-limen,' which 
latter averaged for the four observers with the red 
paper about 1.2°. That is, a sector of 1.2° of red 
when mixed with 358.8° gray causes a just percep- 
tible appearance of color. It appears from the fore- 
going that the estimated number of least perceptible 
differences in saturation of the red pigment under 
the conditions of the experiment is about 100. 

Another group of experiments was made with 
nine observers using red, yellow, green, and blue 
colored papers and their corresponding grays. These 
measurements were made for each eye separately 
and for binocular vision. Geissler places no great 



COLOR AND VISION 129 

emphasis upon the absolute values of the results 
because of the lack of sufficient observers and the 
incompleteness of the investigation at present. How- 
ever, it is of interest to give the mean results for the 
nine observers. The averages for binocular vision 
were, as a rule, lower than for monocular vision. 
The results for all observers for monocular and binoc- 
ular vision gave as the mean limenal values of color 
saturation for red, yellow, green, and blue respec- 
tively, 2.23°, 5.81°, 7.19°, and 2.99°. That is, these 
values represent the smallest increments required 
to distinguish between * color and no color.' The 
comparison was made between a gray disk and a 
concentric disk of the same gray in which the color 
was introduced. The brightnesses were previously 
equated by means of a flicker photometer. The 
colored papers differed from each other in brightness 
and saturation, which appeared to have an influ- 
ence on the values of just perceptible saturation-dif- 
ference. Since the green requires a limen three 
times as great as that of red it appears to Geissler 
that it is reasonable to assume that its saturation is 
only one-third as great as the red and about one- 
half that of the blue. These figures agree approxi- 
mately with a number of estimates of saturations 
made by some of the observers, but in the absence 
of sufficient data little emphasis is given to this point. 
Experiments with a practically color-blind subject 
indicated that his limenal values were extremely 
high, being 37°, 18°, 140°, and 8.25° respectively for 
the red, yellow, green, and blue papers. No analysis 
of his defect was made. 

There appears to be a need for a further explora- 
tion in this interesting field. 

37. Visual Acuity in Lights of Different Colors. — 



130 eOLOR AND ITS APPLICATIONS 

As has already been shown the eye is not achromatic; 
that is, rays differing in wave-length do not come to 
a focus at the same point, with the result that the 
image of an object illuminated by light of extended 
spectral character is not sharply defined upon the 
retina. Louis Bell ^^ compared the acuity of the eye 
or its ability to distinguish fine detail in tungsten 
and mercury arc lights and obtained results indi- 
cating an advantage for the latter illuminant. This 
he attributed to the more nearly monochromatic 
light emitted by the mercury arc. It will be remem- 
bered (Fig. 4) that the preponderance of visible rays 
is confined to a rather narrow wave-length range in 
the yellow and green regions of the mercury spectrum. 
The author ^^ verified these results and extended 
the investigation to lights of the same color but dif- 
fering in spectral character. By using the lights 
whose spectra are shown in Fig. 17, no difficulties 
of color photometry were encountered. Screens 6, 
c, dj used with a vacuum tungsten lamp operating 
at 7.9 lumens per watt yielded lights of the same 
yellow color but of different spectral character. Like- 
wise screens e and /yielded two green lights, one 
purely monochromatic (mercury green line), and the 
other a green of extended spectral character. The 
data, except in case 4, Table XII, were not obtained 
as usual by using fine detail at the limit of discrimi- 
nation but instead, in terms of equal * readability' 
of a page of type, which proved after some practise 
to be a rather definite criterion. Some such method 
should be applicable to many practical investiga- 
tions in lighting, for it renders results in terms of 
a criterion which, although apparently indefinite, is 
found to be quite definite and one which renders 
results full of significance. The results for the 



COLOR AND VISION 



131 



TABLE Xn 
Relative Illumination for Equal Readability 



Case 


Source 


Screen 


Color 


Approx. 
foot candles 


Relative 
illimiination 


1 


Mercury arc 
Tungsten lamp 


f 
e 


green line 
green 


2.0 


1.00 
1.75 


2 


Tungsten lamp 
Ttmgsten lamp 


d 
c 


yellow 
yellow 


4.0 • 


1.00 
1.33 


3 


Sodium lines 
Timgsten lamp 


none 
c 


yellow lines 
yellow 


0.5 


1.00 
1.66 


4 


Mercury arc 
Tungsten lamp 


f 
e 


green line 
green 


0.6 


1.00 
5.10 



author's eye are shown in Table XII and are given 
in terms of the relative illumination required for 
equal readability of a page of type. In case 4 an 
acuity object proposed by H. E. Ives ^^ and devel- 
oped by P. W. Cobb was used. Here the criterion 
was the ability to perceive fine lines at the limit of 
discrimination. 

Other observers obtained results of a similar 
nature with the same apparatus. No stress is laid 
upon the accuracy of the absolute values, but it is 
conclusively evident that monochromatic light is 
superior for discriminating fine detail. Later it was 
shown, 1^ as was expected from the foregoing, that 
monochromatic light was superior to daylight for 
discriminating fine detail. In this case the Ives 
acuity object was viewed against a white magnesium 
oxide surface which was illuminated to an intensity 
of 10 meter candles (approximately one foot candle). 
The visual acuity on the Snellen scale was found to 
be 1.28 and 1.11 respectively for daylight, and mono- 



132 COLOR AND ITS APPLICATIONS 

chromatic green light of equal intensities and results 
for tungsten light and daylight were practically iden- 
tical. Another experiment showed that for visual 
acuity of 1.28 on the Snellen scale the intensity of 
illumination with daylight or tungsten light was 
nearly three times that required for the same visual 
acuity with monochromatic green light. As the 
brightness of the background was increased it ap- 
peared that the difference in visual acuity under a 
given illumination of tungsten light and monochro- 
matic light decreased. 

The superior defining power of monochromatic 
light having been demonstrated, it is of interest to 
learn if there is any difference in the defining power 
of monochromatic lights of different colors. Dow ^^ 
measured visual acuity in light of different colors 
using electric lamps screened with colored media 
and arrived at the conclusion that the blue-green 
region of the spectrum showed greater defining 
power. Ashe ^^ used red, green, blue and clear 
glasses with incandescent lamps and found visual 
acuity least for the red and increasing in the order 
green, blue and clear glass for the same illumina- 
tion; however, the data were too incomplete to war- 
rant any definite conclusions. Loeser ^^ used red, 
green, and white papers on which black characters 
were printed. The papers were brought to equal 
brightness and visual acuity was determined by noting 
the greatest distance at which the observer could 
distinguish the details on the papers. He found 
acuity greater for green light than for red light, and 
also that the characters on the white card could be 
distinguished at nearly as great a distance as those 
on the green card. A serious defect in this method 
is the fact that, the distances not being constant, the 



COLOR AND VISION 



133 



change required in the accommodation of the eye 
complicates the results. Uhthoff ^o determined visual 
acuity in monochromatic lights of different wave- 
lengths, but gives no data on the relative brightnesses 
of the colored lights. A serious defect in most of 
the above work is the fact that the lights were neither 
monochromatic nor did their spectra extend over 
equal ranges of wave-lengths. The same criticism 
is applicable to the work of Rice,^! who performed an 
extensive investigation of the problem. 

In order to determine visual acuity in mono- 
chromatic lights of different colors at ordinary bright- 

^fC eld 



■0 



n 







j^ 



Q> 



-%^ 



\gnmrTv/ 



Fig. 68. — Apparatus for determining visual acuity in monochromatic lights. 

nesses, the author 22 devised the apparatus shown 
diagrammatically in Fig. 68. The lines of the acuity 
object/^ c, having a highly illuminated ground glass 
background, cf, were focused crosswise on the slit 
of a Hilger wave-length spectrometer by the lens, /. 
On looking into the eyepiece these lines were viewed 
against a background whose color depended upon the 
position of the prism, the wave-length being indicated 
on the drum, n. On the pointer in the eyepiece was 
mounted a minute piece of magnesium sulphate, mmj 
at an angle leaning away from the eye at the top. 
This was illuminated by means of the frosted tung- 



134 COLOR AND ITS APPLICATIONS 

sten lamp, y, the light being reflected downward by 
the mirror, o. Slides hh controlled the width of the 
photometric field, and an artificial pupil, &, was placed 
in front of the eyepiece. The drum, k^ controlled 
by means of a belt the size of the lines of the test 
object which was read from drum e. The photo- 
metric balance was made, in the case of each mono- 
chromatic light used, by balancing it against the 
white surface mm^ the lines of the acuity object at 
the time being too small to be visible. A feature 
of this acuity object which is essential for such a 
use is that the average brightness of the object is 
constant regardless of the width of the lines. Of 
course in making the photometric balance the un- 
certainties of color photometry are present, but these 
are not of much importance in this investigation, 
because visual acuity changes very slowly with 
change in brightness of the object at the illumination 
used; therefore, a large error in the photometric meas- 
urements would cause but a slight error in the visual 
acuity measurements. The brightness of the photo- 
metric field as seen by the eye through the artificial 
pupil was equivalent to the brightness of a white 
surface illuminated to an intensity, of 4.2 foot candles. 
After the photometric balance was made by varying 
the current through the large lamp illuminating the 
test object, the lamp, 7, was extinguished and a series 
of acuity settings were made by varying the size of 
the lines. The results obtained are shown in Fig. 
69. Curves a, c, represent extreme series made by 
the author showing the fluctuation in the ability of 
the eye to distinguish fine details, and h is the mean 
curve of a great many observations. Curves d and 
e represent single series of observations (ten read- 
ings at each point) made by two other observers. 



COLOR AND VISION 



135 



In every case the observer was permitted to focus 
the instrument. These data indicate an advantage 
in the defining power of monochromatic yellow light 
over other monochromatic lights of equal brightness. 
In order to extend the observations into the violet 
end of the spectrum, the test object was illuminated 
by means of a mercury arc. The mean results for 
each of two observers are shown in Fig. 70, for three 
mercury lines. Curve E was combined with curve 
b in Fig. 69 (obtained by the same observer) which 



0-40 0.44 0.4a 0.52 Q56 0.60 0.64 0.65 

Fig. 69. — Visual acuity in monochromatic lights of equal brightness. 

extended the latter as indicated. This investiga- 
tion indicates that monochromatic lights differ in 
their defining power and that yellow monochromatic 
light is superior to others in this respect. It was 
also found that for a given change in brightness of 
the test object the change in visual acuity was least 
for yellow monochromatic light than for light of any 
other spectral hue. 

A striking experiment illustrating the effect of 
spectral character of light on visual acuity is given 
below. The test-object was viewed through an ethyl 
violet screen (purple under the illumination from a 
tungsten lamp) and visual acuity settings were made. 
After obtaining the mean of a series of observations 



1.4 


















^^ 






















^ 


_r^ 




— ■ 






\s. 


s. 














^'l 


^ 


.^^. 




^-^ 


\ 


"*-*- 


\ 


^ 


a 
b 


I 2 


t-^ 


— 












x^' 


y 










\ 


k 




N 


d 


JO 






.-" 
















\ 


-^. 




e 

































136 



COLOR AND ITS APPLICATIONS 



a yellow screen was also placed before the eye. This 
screen absorbed the blue and violet rays transmitted 
by the purple screen, thus reducing the illumination 
at least 50 per cent. Notwithstanding this reduction 
in illumination visual acuity noticeably increased. 
In place of the yellow screen was now substituted a 
blue screen which absorbed the red rays transmitted 

1.4 



I.-5 



1.2 



3.1.1 

< 

< 

I 1.0 

> 

1-^ 



J 



.^ 





















































■ 








































^' 


-<, 


X 




























/ 




























^ 


y^ 


y 


























^ 


/ 


y 


1^ 
























^ 


x^ 




y 
/' 
























,^ 


^ 






y 
























■ • 






y 


y 




























V 


/ 






























V 


^' 
































::S: 

1 




























V 






























' ■% 






















-J 






^ 






















AD- 
1 
































II 



042 



0.46 



0.50 



0.54 



0.58 



Fig. 70.' — Visual acuity in the mercury spectrum, the lines being reduced to 

equal brightness. 



by the purple screen, the resulting light being blue. 
Again visual acuity increased, notwithstanding the 
reduction in brightness. This experiment strikingly 
demonstrates the influences of chromatic aberration 
and spectral character of light on the ability to dis- 
tinguish fine detail. 

It is interesting to note some results on the legi- 
bility of colored advertisements. Le Courrier du 
Livre ^3 reported the legibility of various combinations 



COLOR AND VISION 137 

for reading at a considerable distance, the most leg- 
ible print being black on a yellow background. The 
order of merit was found to be as follows: 

1. Black on yellow 8. White on red 

2. Green on white 9. White on green 

3. Red on white 10. White on black 

4. Blue on white 11. Red on yellow 

5. White on blue 12. Green on red 

6. Black on white 13. Red on green 

7. Yellow on black 

It is noteworthy that in this list the customary 
black-on-white combination is sixth on the list. 
These results are interesting, although perhaps not 
final, owing to the many variables that enter such- a 
problem. 

38. Growth and Decay of Color Sensations. — 
Many investigators have studied the problem of the 
effect of time of exposure and intensity of the stim- 
ulus on the growth and decay of luminous sensations. 
It has been noted (# 14) that colors are seen on rotat- 
ing, at a proper speed, aTdisk composed of black and 
white sectors. It appears that this is due, in part 
at least, to the difference in the rate of growth and 
decay of the various color sensations excited by 
white light. Of the work in this field, that of Broca 
and Sulzer24 is especially comprehensive. They com- 
pare the brightness of a white screen illuminated by 
a light of short duration with that due to a standard 
steady light. Some of their results which are plotted 
in Fig. 71 show that, excepting for lights of low inten- 
sity, the luminous sensation * overshoots' its final 
value; that is, the maximum luminous sensation is 
passed a comparatively short time after the begin- 
ning of the exposure and that the luminous sensa- 
tion reaches a steady value less than the maximum 



138 



COLOR AND ITS APPLICATIONS 



only after the elapse of an appreciable fraction of a 
second (depending more or less upon the intensity). 
The numbers on the curves indicate the final steady 
value of the various stimuli. Their data obtained 
with colored light, plotted in Fig. 72, indicates that 
under the stimulation of blue rays the luminous 



80 
75 

y 70 

o 
5 65 

O 

i 60 
lij 

13 50 

jz 

S 45 
I 40 



35 



i 50 

u. 
<^ 25 























, ,. 




































































i 
























































\ 




























1 


\. 




























\ 






























\ 
























r 


N 






— 




122^ 


_ 










/ 










^ 






/PfT 












/ 



























r 










^_ 






^ 


\5Li 






J26 




^ 


y^ 






J^^J 


ux 






/x 




-m 














^ 


. 










/6.2 Lux 










^ 










1 










""^^ 



005 



0.1 



0J5 0.20 

SECONDS 



0.25 



0.3 



Q35 



Fig. 71. — The growth and decay curves for white light sensation. (Broca 

and Sulzer.) 

sensation overshoots very much more than in the 
case of red or green light, the latter showing the 
least overshooting. 

In studying the growth and decay of color sensa- 
tions in connection with the flicker photometer ^^ some 
data of interest here were obtained. Red and blue- 
green lights, practically complementary, were matched 
by the ordinary direct comparison method of photom- 



COLOR AND VISION 



139 



etry. These were then separately flickered against 
darkness by means of a rotating disk with equal open 
and closed sectors. The maximum brightness of 
the flickering light was compared with a steady 
brightness of the same color for a large range of 
flicker frequencies. The data is shown in Fig. 73, 



0.25 




UJ 

g 100 

UJ 

tl 







J— ■ 






m 




- 








/ 


/^ 


Gn 


U 


SI 


-05 
S^ 








u 


r 








25 


26 


' 




^~ 














. 



0.05 



0.1 0.15 

StCONDS 



0.2 0.25 
Fig. 72. — The growth and decay curves of color sensations. 



the initials R and G representing the red and blue- 
green lights and the subscripts, high and lower in- 
tensities. The intensities used were those ordinarily 
considered satisfactory in photometry as is indicated 
by the frequency in cycles per second required to 
cause flicker to disappear. It will be noted that the 
colored lights were alternated against darkness, the 
steady values of the colored lights (sectors open) as 



140 



COLOR AND ITS APPLICATIONS 



determined by the direct comparison method being 
represented by unity on the relative brightness scale. 
The flicker of Gl, /?l, C^h? and i?H completely dis- 
appeared at frequencies corresponding respectively to 
Aj B, C, and D. 

Next red and blue-green brightnesses equivalent 
to the foregoing were placed so that on one side of 



1.4 



1.2 



1.0 



0.8 



a: 0.6 



0.4 































\ 


























Y^M 


























\ 
























0, 


\ y 
























■ 




L 
























Vi 


K 
























'\ 




V 






















V 


v^. 


^^\ 






















1 


^^ 


s. 


•^ 


V 






















^^> 




-> 


^^^* 
























t 




B 








1 


"D" 
1 



2 4 6 8 10 12 14 16 18 20 22 24 2& 
FLICKER FREQUENCY (CYCLES PER SECOND) 

Fig. 73. — Showing the maxima attained by flickering lights at various 
frequencies. 



the photometer field a red light flickered on a steady 
blue-green field and vice versa on the other side. 
This was done by means of identical sectored disks 
(180° opening) placed one on each side of the 
photometer. On one side a disk intercepted the 
blue-green light and on the other the red light was 
intercepted. On increasing the speed of rotation of 
the disks (which were fastened to the same shaft) the 
side on which blue-green light flickered upon a steady 
red field became quiescent long before the flicker 



COLOR AND VISION 



141 



disappeared on the other side. At all times when 
flicker was visible the side upon which red flickered 
on a steady blue-green field appeared to attain higher 
maximum values of brightness and to be more agi- 
tated. The brightnesses on either side were later 



24 




'\ 




























/ 
/ 


\ 




























22 


/ 






























1 
/ 






























UJ 


1 






























1 






























5 1.8 


1 
1 




\ 


























CD 


1 

1 




\ 


























r 
gl.6 

>< 


1 






\ 
























1 

1 






\ 
























uJ 1.4 


1 
1 






\ 
























1/ 


\ 




\ 


f 
























\ 




\ 




\ 






















1 




\ 


\ 




\ 




















1.0 


1 
1 






> 


\ 




\ 


























S 


\ 




X 


N^ 














Q6 














N 




s 


\ 




























^-%. 


,^ 


^"^'t^'*^- 





















5 12 16 20 

FLICKER FREQUEIiCY 



24 



2& 



Fig. 74. — Showing the maxima of sensations produced by flickering red light 
on a steady green field (R), and vice versa (G). 

measured separately against a steady white light 
(there being little color difference excepting at low 
speeds) throughout a wide range of frequencies. 
These results are shown in Fig. 74, R and G indi- 
cating that red and blue-green were respectively the 
flickering components. The steady value reached at 



142 COLOR AND ITS APPLICATIONS 

a high frequency is 0.75, unity being taken as the 
steady value at zero speed with the sectors open. 
The latter intercept only one of the two components 
which make up the brightness on either side; there- 
fore, the sectors being of 50% transmission, the 
final value at a high frequency of alternation is 0.75 
of the original steady value with sectors open. Of 
course these experiments involve the measurement 
of the brightness of sxxrfaces differing in color, but 
it is this problem that was involved in the study. All 
steady brightnesses were chosen equal as measured 
by an ordinary direct-comparison photometer. While 
these effects of different rates of growth and decay 
of color sensations are operative when there is an 
apparent flicker, evidence points to the disappearance 
of such influence upon the brightness of a mixture 
of colored light by alternately presenting the colored 
stimuli when the rate of alternation is so high that 
flicker has disappeared. For instance the foregoing 
red and blue-green lights were mixed by alternating 
them by means of a sectored disk (50% opening) 
and also by directly superposing the steady lights. 
The former mixture was found to be just one-half 
as bright as the latter, within the slight possible errors 
of the experiment. There was no color difference 
present in this experiment so the photometric data 
is correct to within one per cent. Other evidence 
of the same kind was obtained by comparing two 
yellow lights of the same hue, but differing in spectral 
character, by means of both the flicker and direct 
comparison methods of photometry. Identical results 
were obtained by the two methods. These results 
were also confirmed by comparing tungsten light 
by the two methods with a light of the same hue 
consisting of red and blue-green lights. (See # 55.) 



J 



COLOR AND VISION 143 

Talbot 2^ long ago expounded the law that a 
sectored disk rotating at high speed transmitted 
light in direct proportion to the angular openings of 
the sectors. This law has been stated by Helmholtz^^ 
as follows: *If any part of the retina is excited with 
intermittent light, recurring periodically and regularly 
in the same way, and if the period is sufficiently 
short, a continuous impression will result which is the 
same as that which would result if the total light re- 
ceived during each period were uniformly distributed 
throughout the whole period." Plateau,^^ Kleiner, ^^ 
Weideman and Messerschmidt,^^ Ferry,^! Lummer and 
Brodhun,32 Aubert,^^ Hyde,^^ and others have inves- 
tigated the problem, and have generally agreed that 
the law holds for white light. Fick concluded that 
it holds only at moderate intensities and Ferry veri- 
fied the law for white light but found discrepancies 
when one side of the photometer field was bluish as 
compared to the other side. Hyde, after a thorough 
investigation of the problem, concluded that the law 
holds within the accuracy of the work (about 0.3%) 
for the range of sectors used by him, namely from 
288° to 10° in opening. He further concluded that 
the law held for red, green, and blue lights within 
the accuracy of precision photometric apparatus, and 
found that when a color difference existed on the 
two sides of the photometer field no appreciable 
deviation from the law was observed. The author has 
had many opportunities to test the law for colored 
lights and found no deviations within the accuracy 
of the experimental work, which was usually well 
within one per cent. The sectored disk, there- 
fore, affords a means of altering the intensity of 
colored light in definitely measurable amounts. 

Lights of very short duration are perceptible if 



144 COLOR AND ITS APPLICATIONS 

intense enough. For instance, a lightning flash as 
short as one-millionth of a second is visible and by 
rotating mirrors flashes of light as short as one eight- 
millionth of a second have been perceived. Blondel 
and Rey ^^ studied the perception of lights of short 
duration at their range limits. Bloch ^^ had pre- 
viously contended that the excitation necessary for 
the production of the minimum sensation was per- 
ceptibly constant and proportional to the product of 
the brightness and the duration. Charpentier ^^ veri- 
fied the law within certain limits. Blondel and Rey 
conclude that Bloch's law can be applied only to 
intense lights of very short duration. After a very 
extended investigation they deduce a simple law, 
(B — Bo) t = aBoy where Bo is the minimum per- 
ceptible brightness of the field, t the duration of the 
stimulus in seconds, and a is a constant of time equal 
to 0.21 second. They show by simple integration 
one can deduce from the law of the flashes which 
are not uniform, their range and the intensity of the 
equivalent constant light from the point of view of 
range, 

fhdt 



a + U-ti 



where 4 represents the photometric intensities of 
the luminous points measured in a horizontal section 
of the beam and referred to unit distance. They 
conclude by taking into consideration the curves of 
sensation of Broca and Sulzer ^^ *that the maximum 
utilization of a source of light must demand short 
flashes without its being necessary to take any 
notice of an inferior limit of the period of the signals, 
except in the case of telegraphic signals. It more- 



COLOR AND VISION 



145 



over suffices that the period of the flash, ^2-^1, should 
become a negligible quantity in the presence of the 
constant a, in order that a maximum efficiency may 
be assured.' 

On alternating a given brightness with darkness 
by means of a sectored disk with 50% openings, a 
violent flicker is evident at low speeds; however, 
there is a certain minimum frequency, called the 
critical frequency, at which the flicker just disappears. 
The critical frequency depends upon the intensity of 
illumination or brightness of the observed field and 
increases with the brightness. Porter ^s has found 



i 




LOGARITHM OF BRIGHItlESS 



Fig. 75. — Showing the relation between brightness and critical frequency for 

colored stimuli. 



that the relationship, / = a log / + 6, holds for white 
light where / is the critical frequency, /, the illumi- 
nation, and a and b are constants; that is, there is 
a straight line relation between the critical frequency 
and the logarithm of illumination. The constant, a, 
has two values, one for brightnesses above those 
resulting from illuminations on a white surface greater 
than about 0.25 meter candles and one below. It 
is thought by adherents to the von Kries * duplicity 



146 COLOR AND ITS APPLICATIONS 

theory' (#48) that this point of abrupt change in slope 
corresponds to the change from cone to rod vision. 
Haycraft^^ has studied the critical frequency for 
spectral lights, but the results are complicated, be- 
cause the intensity of the various rays was not con- 
stant throughout the spectrum. Ives^'^ studied the 
relation between critical frequency and brightness for 
various spectral rays and obtained results which he 
expresses diagrammatically as shown in Fig. 75, the 
logarithm of brightness being plotted against critical 
frequency. It is noted that the *red' curve shows 
no change in direction at low intensities. The blue 
curve changes from a diagonal to a horizontal straight 
line; that is, at low illuminations the critical fre- 
quency becomes constant for blue light of various 
feeble intensities. Intermediate curves represent spec- 
tral colors between red and blue. It is significant 
to note that the slopes of the curves are different for 
the higher illuminations, the *blue' slope being 
steeper than the *red' slope, which indicates that 
the Purkinje phenomenon is operative. 

The author ^^ has shown that the critical frequency 
depends upon the wave form of the stimulus or the 
contour of flicker. Some of the data for white light 
is shown in Fig. 76. In cases a, &, c, the maximum, 
minimum, and mean cyclic illumination were re- 
spectively the same. A difference in critical fre- 
quency was obtained throughout a wide range of 
illumination, the critical frequency being higher the 
greater the period of darkness in a given cycle. 
This also appeared to hold for colored lights, but no 
extensive study has yet been made. 

39. Signaling. — The chief requisite of a colored 
light for signaling purposes is high intensity, because 
its range depends largely upon this factor. This 



COLOR AND VISION 



147 



precludes the use of very pure colors owing to low 
intensities obtainable in practise, and for this reason 
signal glasses are a compromise between saturation 
of color and transparency. As is seen by the redness 
of the setting sun, red rays are less absorbed by 
smoke and dust in the atmosphere than the blue rays, 
therefore, a red signal should have a greater range 
than a blue signal through a smoke and dust laden 




20 50 40 50 



RELATIVE MEAN BRIGHTNESS (LOGARITHMIC 
SCALE) 



Fig. 76. — Effect of contotir of flicker on critical or vanishing-flicker frequency. 



atmosphere. Churchill ^^ quotes results obtained by the 
Geheimrat Koerter of the German Lighthouse Board on 
the selective absorption of artificial fog. The results 
indicated that red rays were absorbed to a greater 
degree (about 20% more) than blue rays. These 
results indicate that the selective absorption of clouds 
plays no part in the shifting of the color of the setting 
sun toward red. On the other hand the German 
Government in about 1900 found that an installa- 
tion of arc lamps in a lighthouse on the island of 



148 COLOR AND ITS APPLICATIONS 

Heligoland had a lesser range in a fog than a kero- 
sene light of only one-hundredth the candle-power. 
The former contains a predominance of blue rays in 
comparison with the latter, which from the foregoing 
tests in artificial fog apparently suffered less absorp- 
tion. 

The German Lighthouse Board of Hamburg in 
1894 carried out an extensive series of tests on the 
range of signal lights with the following results as 
presented by Churchill, where R represents the range 
in miles and / the candle-power. 

For white light in clear weather R = 1.53a/I 
For white light in rainy weather R = 1.09\/l 
For green light in clear weather R = 1.6 v^ I 

M. Busstyn ^^ estimates the range of red light as 
R = l.SVi. The spectral character of the illuminant 
of course has an important influence on the color 
of the signal glass. 

It is interesting to note that on a certain modern 
battleship a lighting system of blue lamps has been 
installed for use at night when in action. The reason 
given for installing blue lights is that they are in- 
visible to the enemy. No information was obtain- 
able as to whether the short range is due to the 
faintness of the blue lights or to a supposed lower 
range for blue than for yellow light of equal intensity. 

The Railway Signal Association (1908) after exten- 
sive tests arrived at the conclusions expressed in 
Table XIII for the effective range of the principal 
signal colors under average weather conditions. The 
colored glasses are assumed to be used with the 
customary semaphore lamp and lens. 

Paterson and Dudding ^^ have performed some 
interesting experiments on the visibility of point 



COLOR AND VISION 



149 





TART.E Xm 




Color 


Effective range 
(miles) 


Approx. transmission 
coef. of glass in service 


Red 


3 to 3.6 

1 to 1.5 
2.6 to 3.0 
0.6 to 0.76 
0.6 to 0.75 

2 to 2.5 


0.20 


Yellow 


.35 


Green 


.17 


Blue 


.03 


Purple 


.03 


Lunar-white 


.16 







sources made by placing plates containing minute 
apertures before a wax flame. While most of their 
work was done indoors at distances as great as 650 
feet, an experiment on the visibility of the light from 
tungsten and carbon incandescent lamps over ranges 
which extended more than a mile showed no differ- 
ence in the carrying power of these lights on a clear 
night. They established the theorem that the visi- 
bility of a point source is proportional to the candle- 
power of the source and to the inverse square of 
the distance. They also found that the visibility 
is independent of the intrinsic brightness for sources 
subtending less than two minutes of arc. In this 
connection it might be noted that they assumed a 
point source to be one whose linear dimensions sub- 
tend an angle at the eye less than the resolving 
power of the eye, i.e. about 30 seconds of arc for 
a mean wave-length of 0.5/>i and pupillary aperture of 
4.5 mm. They found that the visibilities of red and 
green lights in clear air were closely proportional to 
the inverse square of the distance. On slightly 
illuminating the field surrounding the point source 
there was a loss in visibility of about 10% for a red 
light, 15% for a white light, and 18% for a green 
light, all being of equal candle-power. Their method 



150 



COLOR AND ITS APPLICATIONS 



of determining the candle-power of the red and green 
lights in the latter experiment was to determine the 
visibility of the unknown in terms of the visibility 
of a white light of known candle-power assum- 
ing the visibility porportional to the candle-power 
and the inverse square of the distance. Their unit 
of visibility was equivalent to the visibility of a point 
source of one candle-power at 1000 meters distance, 
and they state that the lowest visibility considered 
desirable was 0.12 of this unit. Their results for 
white light agree fairly well with those obtained by the 
Deutsche Seewarte of 1894, as is shown in Table XIV. 

TABLE XIV 



Range 


(Deutsche Seewarte) 
Candle power re- 
quired in clear 
weather 


(Paterson & Dudding) 

Computed from 

results of 

experiment 


1 sea-mile (1855 meters) 

2 sea-miles (3710 meters) 

5 sea-miles (9275 meters) 


0.47 
1.9 
11.8 


0.41 
1.6 
10.0 



By using artificial pupils they found little evidence 
of any influence of the spherical aberration of the 
eye on the visibility of point sources. They showed 
by using positive lenses that a green light equivalent 
to a point source was greatly dimmed relatively to a 
red light of similar dimension, which they attribute 
to the chromatic aberration of the eye. They con- 
clude that unless an observer has sufficient accom- 
modation available to focus properly a green light at 
infinity the latter will appear dimmed in proportion 
to the amount this image is out of focus. This is 
not so likely to occur with red light, because images 
of this color do not require as much accommodation 



COLOR AND VISION 151 

in order to focus them on the retina. A purple 
roundel at some distance sometimes appears red in 
the center with blue diverging from it, which is attrib- 
uted to chromatic aberration of the signal lens. 

It is hardly necessary to state the importance of 
tests for color-blindness of eyes engaged in dis- 
criminating colored signals. Numerous test methods 
have been devised, but one that has been used very 
much is the Holmgren test, conducted with colored 
skeins of wool. 

In the choice of signal colors the four most dis- 
tinctive are red, yellow, green, and blue. To these 
can be added white (clear) and purple. The 
blue and purple owing to their low intensity are 
suitable only for short-range signals. Results by 
Churchill on the reaction times, or the intervals re- 
quired to distinguish and name signals of different 
colors, were in general in the following order. Red 
was recognized and named in the shortest time, 
green ranked next, then yellow, and lastly white, 
which required the longest interval for recognition. 
The relative length of the time interval varied with 
different subjects, but the order given was found to 
be generally the case. Of course the spectral char- 
acter of the illuminant has an important influence on 
the color of the signal glass. 

40. Other Uses for Colored Glasses. — It is well 
known that dust and smoke (and very likely fog) 
scatter the visible rays of short wave-length more 
than those of longer wave-length. For this reason 
it is contended by some that, if the blue and violet 
rays are subtracted from white light, the remaining 
light (yellow in color) will enable an observer to see 
further than the total light. It is of interest to inquire 
further into the matter. In the case of the search- 



152 COLOR AND ITS APPLICATIONS 

light, if the operator wishes to see a distant object 
in a fog he is required to look through an illuminated 
veil caused by scattered light, which results in de- 
creasing the ability to distinguish the object. If 
it is true that the blue and violet rays are scattered 
more by the fog particles, the luminous veil would 
become more annoying for lights containing relatively 
greater amounts of blue rays. Rough quantitative 
tests by the author, employing auto lamps with para- 
bolic reflectors, indicated that with yellow light, which 
was less intense than the total light by the amount 
of light absorbed by the yellow screen, objects in a 
fog could be seen more clearly than with the total 
light. There is another view-point, namely that of 
the person who desires to distinguish a light signal 
at a considerable distance. Here again an illumi- 
nated veil relatively near the light source if visible 
is likely to decrease the visibility of the signal light. 
This point, however, requires investigation. 

Based on the foregoing principle many patents 
have been obtained for methods of eliminating the 
violet rays. Colored glasses, gold-plated reflectors, 
fluorescent glass reflectors, etc., have been employed, 
but all for the same object. A noteworthy problem 
of projection arises with the carbon arc search-light. 
In order to obtain a beam of parallel light by means 
of silvered reflectors the area that emits light must 
be small and be located at the focus of a parabolic re- 
flector. With high-amperage arcs an appreciable 
portion of the light is emitted by the arc flame of 
relatively large area as compared with the crater 
of the positive carbon which is located at the focus of 
the parabolic reflector. The light from the arc flame 
has a decided violet tinge as compared with the light 
from the crater, and furthermore, being out of the focus 



COLOR AND VISION 153 

of the parabolic reflector, its light is not * paralleled,' 
but escapes in a cone of relatively large angle. By 
using a yellowish glass in the aperture of the search- 
light, this light of a bluish tinge is greatly reduced in 
comparison with the reduction of the yellower light 
from the crater, thus decreasing the possible annoy- 
ance due to the * luminous veil.' The same result 
would be obtained if the * look-out' wore yellow 
glasses. In the case of a very powerful searchlight 
such a glass in the aperture probably would be broken 
by the rise in temperature due to its absorption of 
radiant energy. If it were inconvenient for the look- 
outs to wear the yellow glasses before their eyes 
there would be some virtue in the gold-plated re- 
flector which would reduce the amount of blue and 
violet light in the reflected light. However, in all 
such cases there is a cone of light which escapes 
directly without being altered by selective reflection. 
In the application of the foregoing principle to auto 
headlights or to any projectors employing electric 
incandescent lamps, any possible objectionable effect 
of the excessive scattering of violet and blue rays 
can be overcome by incorporating the yellow glass 
directly in the lamp bulb, by applying a yellow color- 
ing to the exterior of the bulb, by inserting a yellow 
glass in the aperture of the reflector, or by wearing 
yellowish glasses before the eyes. An interesting 
case is found in a fluorescent glass reflector (silvered 
on the back surface) which absorbs most of the 
violet and blue rays. One of the claims advanced 
for this reflector is that it utilizes the ultra-violet, 
violet, and blue rays of the incandescent lamp by 
taking advantage of the fluorescent property of ura- 
nium glass which converts these rays into yellow- 
green light; however, these rays constitute a very 



154 COLOR AND ITS APPLICATIONS 

small proportion of the total visible rays in the light 
from electric incandescent lamps ordinarily used for 
such purposes. Furthermore, the fluorescent yellow- 
green light produced by these rays is not * directed' 
by the parabolic reflector, because this light is emitted 
in all directions. On looking at such a reflector it 
appears of a yellowish green tint, but close examina- 
tion shows that the yellow-green fluorescent light 
is emitted by the glass in all directions. Therefore, 
no practical gain in intensity of the directed beam 
results from the conversion of the ultra-violet, violet, 
and blue rays into yellow-green light, because the 
latter is diffusely emitted and the effect of such a 
fluorescent glass amounts to little more than elimi- 
nating most of the violet and blue rays from the 
radiation that is intercepted by the reflector. In 
this case there is also a cone of unaltered light, equal 
in solid angle to that subtended by the aperture of the 
reflector, which escapes * undirected.' This scheme 
appears to have little value, inasmuch as a yellow 
glass in the aperture of the reflector would accomplish 
the purpose in a more satisfactory and simple manner. 
Amber, yellow, and greenish yellow glasses have 
been used successfully for eliminating glare from the 
blue sky. Riflemen have found such glasses of 
extreme value in range shooting and a number of 
sportsman's glasses are available in the market. In 
the case of amber or greenish yellow glasses the 
improved condition of seeing is perhaps largely due 
to the reduction of glare from the blue sky, but also 
in part to an increased defining power due to the 
elimination of blue and violet rays and a relative 
reduction of the extreme ^ed rays (#37). An illus- 
tration of the effect of greenish-yellow glasses ^^ in 
increasing the ease of distinguishing detail is shown 



COLOR AND VISION 



155 



in Fig. 77. An acuity object ^^ (#37) was set up in 
the shade of a building on a clear day and light 
reached the object from at least one-half of the open 
sky. No direct light from the sun reached the eye, 
test object, or immediate surroundings. The author 
who made the observations wore no visor to shield 
the eyes. Only a slight sensation of discomfort was 
apparent before beginning the test; however, as soon 
as acuity observations were begun the glare became 
very evident and rapidly grew painful. Five readings 




5 6 9 12 15 16 

TIMECMIISUTES) 

Fig. 77. — Effect of yellow-green glasses on vision under a bright sky. 



were made first through clear correcting glasses (rep- 
resented by the black dots) and as quickly as pos- 
sible the clear glasses were replaced by yellow-green 
glasses of about 50 per cent transmission for the 
total light and five acuity readings were taken (repre- 
sented by crosses). A decided decrease in discom- 
fort was experienced when wearing the yellow-green 
glasses and, as will be noted, visual acuity is 
higher in this case, notwithstanding the decrease in 
illumination was fully 50 per cent. These glasses 
were again replaced by clear glasses and five acuity 
readings were made. This procedure was continued 
as indicated in Fig. 77. The interval of time required 



156 COLOR AND ITS APPLICATIONS 

to make five readings including the change of glasses 
was the same in each case (being three minutes), 
but the actual time of making the individual readings 
was not noted; therefore, they are plotted at equal 
intervals. While the above conditions are rather 
complex and involve problems worthy of much careful 
investigation, the experiment answered the intended 
purpose in bringing forth several points: (1) Glare 
conditions are not always apparent when the eyes 
are not engaged in serious work such as reading or 
distinguishing fine detail. However, bad lighting 
conditions are readily recognized when the eyes are 
called upon to do such work. (2) There is a rapid 
falling off of visual acuity when the conditions of 
glare are severe. (3) Such a harmless appearing 
light source as a wide expanse of sky can produce 
a very severe condition of glare. The intrinsic 
brightness is very low as compared with artificial 
sources, but the quantity of light is high and the 
image of the sky is spread over a large portion of the 
retina. Its annoyance can be decreased by the use 
of colored glasses, which absorb much of the blue 
light. (4) There was an apparent recuperation of the 
eye during the periods that the yellow-green glasses 
were worn. (5) Notwithstanding the effect of glare 
(when clear glasses were worn) in reducing visual 
acuity the values of the latter when the colored 
glasses were worn remained considerably higher. 
(6) This experiment emphasizes the necessity of 
prolonging acuity readings over a considerable period 
if acuity is to be a criterion of the satisfactoriness 
of illumination conditions. Some of the increase in 
visual acuity when the yellow-green glasses were 
being worn can be accounted for by the nearer ap- 
proach to monochromatism of the light that passed 



COLOR AND VISION 157 

through them. However, conditions indicated that 
the advantage was due very largely to a reduction in 
the glare from the sky because the glasses absorbed 
much of the blue and violet light. Other interesting 
conclusions can be drawn, but the illustration has 
already fulfilled its object in bringing forth the fact 
that glare conditions are very complex and that cog- 
nizance of glare often depends upon the character of 
the activities in which the eyes are engaged. 

Glasses for protecting the eyes from visible, ultra- 
violet, or invisible rays are coming into prominence. 
In considering only the visible rays, colored glasses 
may be combined after the principle of the subtractive 
method of mixing colors (#18, Fig. 20, Plate II). A 
superabundance of violet, blue, or green rays can be 
reduced by the use of red glass. That is, a colored 
glass will greatly reduce rays roughly complementary 
in color. Spectrophotometric analyses, affording data 
such as are shown in Fig. 12, are quite necessary for 
intelligently combining glasses for protecting the 
eyes. Spectrophotographic analysis is a convenient 
means of studying the transmission characteristics 
of glasses in the ultra-violet region and radiometric 
methods are applicable to the infrared region. Ordi- 
nary glass is sufficiently protective against moderate 
amounts of ultra-violet energy. In ordinary lens 
thicknesses it is transparent to about 0.360^, from 
which point it begins to absorb ultra-violet rays, be- 
coming practically opaque to rays of shorter wave- 
length than 0.300)U. Some green, yellow, orange, and 
red glasses are totally opaque to all ultra-violet rays, 
but this cannot be ascertained by a mere visual 
inspection. However, the ultra-violet transmission 
can be roughly determined by means of a quartz 
spectrograph and an iron arc or a quartz mercury arc. 



158 



COLOR AND ITS APPLICATIONS 



On focusing the spectrum of the radiation emitted 
by the quartz arc on a fluorescent material such as 
uranium glass, the various ultra-violet lines will be 
seen owing to their production of fluorescence. On 
inserting a specimen of glass before the slit of the 
spectrograph the region of absorption will be readily 
perceived by the disappearance or decrease in bright- 
ness of various fluorescent lines. The transmission 



100 

90 

80 

5 70 

ai 60 



.50 



/- C/ear lead Class 
Z-Ho.O Smoke 
3- Amethyst 
4- Light Amber 

Ultraviolet 



■'050 034 



5-N0.7 5moke 
6- Ho. 6 Smoke 
7- Medium A mber 
d-Euptios 
d-Akopos 

Visible 





















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Jl, WAVE LENGTH 



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Fig. 78. — Ultra-violet transmission curves of various glasses. 



of glasses in the ultra-violet region ^^ has been de- 
termined by using a wide slit, the spectrum of the 
quartz mercury arc, and a combined photographic 
and photometric method. For qualitative analysis 
an iron arc is a satisfactory source rich in ultra-violet 
rays. Some specimen transmission curves for various 
optical glasses, employed for protecting the eyes 
from ultra-violet energy, are shown in Fig. 78, the 
ultra-violet region being represented to the left of 
the heavy vertical line at 0.40/x. It is interesting 



COLOR AND VISION 159 

to note the difference in ultra-violet absorption of the 
two samples of * smoke ' glass. Of course the absorp- 
tion depends upon the thickness of the specimen and 
its density of color. All the glasses excepting three 
specimens, 3, 5, and 6, transmitted 50% or more of the 
total light from a tungsten lamp. Spectrophoto- 
graphic analyses of various glasses are shown in Fig. 
18. 

In general it is no doubt advisable to use glasses 
as free from color as possible and yet providing 
protection if they are to be worn for long periods. 
Yellow-green glasses when otherwise filling the re- 
quirements appear to distort colors (more commonly 
encountered) less than medium amber. A striking 
instance was found in the lap-welding department 
of a steel mill, where the operators judge tempera- 
ture visually. They became confused when wear- 
ing amber glasses, but found no difiiculty in using 
yellow-green glasses. This brings to mind the fact 
that through a yellow-green glass transmitting only 
a limited region of the spectrum the relation of bright- 
ness and temperature appears practically the same 
as to the unobstructed eye when the luminous sub- 
stance radiates light approximately the same as a 
black or *gray' body. Years ago Crova suggested 
a method of photometry involving this principle (# 54). 
Schanz and Stockhausen, Voege, Crookes,. Parsons, 
and others have studied the subject of protecting the 
eye from harmful rays. Crookes ^^ concludes with 
his associates that the relatively great amounts of 
infrared energy emitted by molten glass is responsible 
for glass-blowers' cataract, although this conclusion 
is questioned by some. He has made an exhaustive 
study of the manufacture of glasses for eye-protection 
and has published the valuable results. 



160 COLOR AND ITS APPLICATIONS 

Colored glasses are often used for bringing out 
certain colored portions of an object in more striking 
contrast with the surroundings. For instance, if a 
black-line drawing be made on blue-lined coordinate 
paper and viewed through a dense blue glass, the 
blue lines practically disappear. If the drawing be 
photographed through this glass the coordinate lines 
will not appear on the negative. In the same manner 
if blue and red appear upon the same background, 
one or the other can be made practically to disappear 
by using a colored screen of exactly the same color. 
Of course the degree of change in contrast will depend 
upon the purity of the colors and the care exercised 
in choosing the colored screen. 

In using field glasses distant vision can be im- 
proved sometimes by the use of a light yellow screen 
which eliminates the blue haze from the visual 
image. In this connection it is well to note also that 
blue rays are normally out of focus at the retina. 
The author has experimented with colored screens 
for use with field glasses for detecting colored objects 
at a distance by altering their contrast with the sur- 
roundings by the use of colored screens. For in- 
stance, a khaki uniform (yellow-orange in color) can 
be made to appear either lighter or darker than the 
green foliage surrounding it by respectively using a 
yellow-orange screen or one of a complementary hue. 
For instance if the ratio of the brightness of a piece 
of khaki cloth to that of a certain green leaf be 
taken as unity under daylight illumination, through 
an ordinary orange filter this ratio became 1.5 and 
through a blue-green filter, 0.7. With care the con- 
trast can be made practically a maximum. In the 
case of objects more striking in color the problem is 
not as difiicult. Whether or not the reduction of 



I 



COLOR AND VISION 161 

brightness more than offsets the advantage of in- 
creased contrast in distinguishing distant objects can 
be solved by actual trial. The point is mentioned 
here to illustrate the possibilities in the use of colored 
glasses as an aid to vision. 

REFERENCES 

1. Physiol. Optik. 1896, p. 140. 

2. Sitz. d. Berliner Akad. 1888, p. 917. 

3. Bui. Bur. Stds. 1907, p. '59. 

4. Lancet, Oct. 2, 1909. 

5. Proc. Roy. Soc. A, 84, p. 464. 

6. Color Scales. 

7. Wein. Sitz. 1906, II a, 115, p. 1. 

8. Bui. Bur. Stds. 6, p. 89. 

9. Bui. Bur. Stds. 9, p. 59. 

10. Trans. I. E. S. 1914, 9, p. 700. 

11. Physiol, d. Netzhaut, Breslau, 1865, p. 138. 

12. Amer. Jour. Psych. 1913, 24, p. 171. 

13. Elec. World, 1911, 57, p. 1163. 

14. Elec. World, 1911, 58, p. 450. 

15. Elec. World, 1910, 55, p. 939. 

16. Elec. World, Dec. 6, 1913. 

17. Lon. Ilium. Engr. 2, p. 233. 

18. Elec. World, Feb. 25, 1909. 

19. Graefe Arch. f. Ophth. 69, p. 479. 

20. Graefe Arch. f. Ophth. 26, p. 40. 

21. Columbia. Cont. to Phil, and Psych. 20, No. 2. 

22. Elec. World, 1911, 58, p. 1252; Trans. I. E. S. 1912, p. 135. 

23. Sci. Amer. Sup. Feb. 2, 1913. 

24. Jour, de Physiol, et de Path. Gen. No. 4, July, 1902; 

Comp. Rend. 2, 1903, p. 977, p. 1046. 

25. Phys. Rev. 1914, p. 1; Elec. World, May 16, 1914. 

26. Phil. Mag. 1834, 5, p. 327. 

27. Physiol. Optik. II, p. 483. 

28. Pogg. Ann. d. Phys. 1835, 35, p. 457. 

29. Pfluger's Archiv. 1878, 18, p. 542. 

30. Wied. Ann. 1888, 34, p. 465. 

31. Phys. Rev. 1895, 1, p. 338. 



162 COLOR AND ITS APPLICATIONS 

32. Zeit. Inst. 1896, 16, p. 299. 

33. PhysioL der Netzhaut, p. 351. 

34. BuL Bur. Stds. 1905, 2, p. 1. 

35. Acad. Sc. Paris, July 3, 1911; Trans. I. E. S. 1912, 7, p. 625. 

36. Comp. Rend. Soc. Biol. 1885, 2, p. 485. 

37. Comp. Rend. Soc. Biol. 1887, 2, p. 5. 

38. Proc. Roy. Soc. 1902, 79, p. 313. 

39. Jour, of PhysioL 21, p. 126. 

40. Phil. Mag. 1912, p. 352. 

41. Meeting Ry. Signal Assn. 1905. 

42. Ann. d . Hydrographie, 1886. 

43. Proc. Phys. Soc. London, 1913, 24, p. 379. 

44. Elec. World, Dec. 6, 1913. 

45. Elec. World, Jan. 15, 1912; Trans. I. E. S. 1914, p. 472. 

46. Proc. Roy. Soc. London, A, 214, p. 1. 



( 



f 



CHAPTER VII 

EFFECT OF ENVIRONMENT ON THE APPEARANCE 
OF COLORS 

41. Colors have been largely treated in other 
chapters as if they were invariable in appearance. 
However, the study and application of the science of 
color is rendered very complex owing to the fact 
that the appearance of a color is so modified by its 
environment. The intensity, spectral character, and 
distribution of the light illuminating it, the adaptation 
of the retina for light and color, the duration of the 
stimulus and the character of the stimulus preceding 
the one under consideration, the surroundings, the 
size and position of the retinal image, the surface 
character of the colored medium, all affect the appear- 
ance of a given color. Thus an analysis that holds 
for a color in a certain environment does not in 
general hold for the identical colored object, viewed 
under other conditions. 

The size of a colored image and its position and 
duration on the retina affects its appearance, owing 
to the variation of sensitivity of the various retinal 
zones. MacDougal^ found that with small colored 
areas (squares from 1 to 16 sq. cm. in area viewed 
from a distance of one meter) the larger areas ap- 
peared more saturated than the smaller. He found 
the saturating effect of increasing the area greatest 
for violet and decreasing in the order, blue, green, 
yellow, orange, red. He even concludes that a color 
field is not fully saturated until it extends over the 

163 



164 COLOR AND ITS APPLICATIONS 

whole field of vision. This can hardly be true, for 
an observer in a room with neutral tint surround- 
ings illuminated with pure red light is not conscious 
of a saturated red color. Similarly if a white paper 
on a black velvet ground be illuminated by a moder- 
ately intense red light it will appear quite unsaturated 
owing to the lack of anything with which to contrast 
it in color. The loss in saturation appears to progress 
with time, no doubt largely due to * adaptation.' 
Whether or not this adaptation is psychological or 
physiological there is a lack of agreement. However, 
the effect of area is of importance, although there is 
much work to be done in this field before definite 
conclusions can be drawn. 

Another experiment of importance in viewing 
colors which is connected with the rate of growth of 
color sensations and, perhaps, to a slight degree, 
with chromatic aberration, is found in viewing a red 
piece of paper on a blue-green ground held at an arm's 
length under a moderate illumination. If the paper 
be moved back and forth without relaxing fixation at 
a point in the plane in which the card is moved, the 
red patch will appear to shake like jelly and will 
appear not to be in the same plane as the blue-green 
paper. Thus there are numerous visual phenomena 
associated with the appearance of colors. 

42. Illumination. — It has already been shown 
that the maximum spectral sensibility of the eye 
shifts toward the shorter wave-lengths at low inten- 
sities (Purkinje effect #4, Fig. 2). Therefore colors 
ordinarily encountered appear to shift in hue under 
low illumination. For example, a green pigment 
appears to assume a more bluish hue as the illumi- 
nation is greatly decreased. On referring to Fig. 2 
it is seen that the relative values of luminous sensa- 



EFFECT OF ENVIRONMENT ON COLORS 



165 



tion produced by equal amounts of radiant energy 
depend upon the wave-length. A colored pigment has 
the ability to reflect certain proportions of the rays 
of various wave-lengths. The latter is a purely physi- 
cal operation which remains invariable regardless of 
the intensity of illumination. However, the relative 
physiologic effect of the different rays change so that 
the maximum luminosity is produced by energy of 
a shorter wave-length at low intensities than at high 
illumination. By multiplying the reflection coefficients 



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0.42 04& 0.50 0.54 058 0.62 

J^O, WAVE LENGTH 

Fig. 79. — Effect of the intensity of illiunination on the appearance of a pigment. 



of a pigment for various rays by the luminosities of 
the corresponding rays at high and low intensities an 
idea of the shift of the dominant hue is obtained. 
This was done for a green pigment by using the 
luminosity curves in Fig. 2 for high {H) and low (L) 
illumination. The results plotted with equal maxima 
are shown in Fig. 79. 

Colors appear more saturated at low than at high 
intensities of illumination. In fact, intense illumina- 
tion causes colors to appear very much less saturated. 
For instance, a deep red object illuminated by direct 
sunlight is painted orange-red by the artist. The 
employment of this illusion is successful in conveying 



166 COLOR AND ITS APPLICATIONS 

to the observer the idea of intense illumination. Simi- 
larly colors appear more saturated when exposed 
only for a very short interval of time. 

. Quality or spectral character of light affects the 
appearance of colored objects very much. Except 
in very special cases a red fabric for example appears 
red because it has the ability to reflect chiefly the 
red rays (# 12, Fig. 12). Such a fabric must appear 
black when viewed under an illuminant which con- 
tains no red rays. This is the case under the light 
from the mercury arc, which contains practically no 
visible rays longer than 0.579^ (yellow). It is a 
fundamental principle that, excepting in special cases, 
a colored fabric cannot appear the same under two 
different illuminants. Therefore two colors that ap- 
pear alike under one illuminant will not match when 
viewed under another illuminant, unless the colors in 
each case* show the same spectral character by spec- 
trophotometric analysis. In other words, because the 
eye is not capable of analyzing a color spectrally, it 
is possible to produce colors which appear the same 
but whose spectral compositions differ. Such a match 
will not in general remain a match under another 
illuminant differing in spectral character. In Fig. 80 
the effect of the illuminant upon the appearance of 
a colored pigment (purple) is shown diagrammatically. 
The relative luminosities (dotted lines) produced by 
the relative amounts of energy (full lines) of cor- 
responding wave-lengths are shown for daylight in 
the illustration on the left. The result as shown by 
the dotted curve is to give to the pigment the appear- 
ance of a blue-purple. However, when this same 
fabric is illuminated by ordinary artificial light of 
continuous spectral character, the excessive amounts 
of energy of the longer wave-lengths and the defi- 



EFFECT OF ENVIRONMENT ON COLORS 



167 



ciency in short-wave energy as compared with day- 
light alter the spectral character of the light reflected 
(or transmitted) by the fabric as shown in the 
dotted curve on the right. The appearance under the 
artificial light is red-purple. It is difficult to distin- 
guish a blue fabric as blue under ordinary artificial 
light owing to the scarcity of blue rays in most of 
the artificial illuminants. Of course a truly mono- 
chromatic pigment (if such existed) would not be 





HooN Sunlight 


^, 


ORDinARY Artificial Light 


/ 






V 


B G Y R 


V 


B G Y R 






Blue -Purple 






Red-Purple 

















Fig. 80. — Illustrating why a purple appears differently under two different 

illuminants. 



changed in hue under various illuminants but would 
be altered in brightness. In the special case where 
no energy existed in the illuminant of the wave- 
length corresponding to that reflected by the mono- 
chromatic pigment, the latter would appear black. 
However, no monochromatic colors are found in 
practise, but if pigments that were practically mono- 
chromatic existed very generally, a greater intensity 
of illumination would very often be required than 
at present, because such colors would reflect very 
little light. Pigments ordinarily encountered re- 
flect considerable light, owing to the fact that they 



168 



COLOR AND ITS APPLICATIONS 



reflect energy throughout an appreciable range of 
wave-lengths. 

The spectral character of an illuminant not only 
influences the hue but also affects the brightness or 
'value' of a pigment. Practically the whole series 
of Zimmerman papers were measured for their rela- 
tive brightness with a reflectometer (Fig. 60) under 
illuminations respectively from an overcast sky and 
from a vacuum tungsten lamp operating at 7.9 lumens 
per watt. The data are given in Table XV, the cata- 



TABLE XV 
Effect of Spectral Character of Light on the Brightness of Colored Papers 





Color of paper 


Reflection Coefficient 




Paper 


Overcast sky. 


Tungsten 

1.25 

w. p. m. h. c. 


R (Tungsten) 
^^*"r (Skylight) 




Red-purple 


0.16 
.14 
.21 
.19 
.38 
.60 
.60 
.67 
.46 
.49 
.32 
.23 
.13 
.14 
.30 


0.23 
.22 
.31 
.24 
.48 
.66 
.65 
.70 
.42 
.45 
.24 
.17 
.09 
.12 
.25 


1.44 


b 


Deep red 


1.57 




Red 


1.48 


d 


Red 


1.22 




Orange 


1.26 


f 

g 
h 


Orange-yellow 

Yellow 


1.10 
1.08 


Greenish yellow 

Yellow-ereen . . 


1.04 
0.91 


i 


Dull green 


0.92 


q 


Saturated green 

Blue 


0.75 
0.74 


Q 


Deep blue 


0.69 


P 
m 


Blue-purple 


0.86 


Gray-blue 


0.83 









logue designations of the papers being found in the 
first column. As would be suspected, the papers 
which have the ability to reflect the rays of longer 
wave-length predominantly appear relatively brighter 
under the artificial light. In other words their reflec- 
tion coefficients are greater for the tungsten light 



EFFECT OF ENVIRONMENT ON COLORS 169 

than for daylight. Those colors having the ability 
to reflect the rays of shorter wave-length predomi- 
nantly, have relatively greater reflection coefficients 
for daylight; that is, they appear relatively brighter 
under daylight illumination. (See Figs. 113 and 117.) 
Thus it is seen that the spectral character of the 
illuminant has a great influence on the appearance 
of a colored object, inasmuch as it influences both 
the hue and brightness (value) very much. 

Owing to the surface character of colored media 
the distribution of light is of some importance in the 
consideration of the appearance of colors. Few pig- 
ments are applied in such a manner as to be per- 
fectly diffusing, therefore some light is specularly 
reflected without having penetrated the pigment. 
This light is unchanged by selective absorption and 
dilutes the light that is colored by penetrating the 
pigment and being selectively reflected. That is, 
when the light is distributed in such a manner that 
an appreciable amount is specularly reflected into 
the eye of the observer the color appears less satu- 
rated. In the extreme case of high specular reflection 
the pigment appears the same as a gray. A striking 
illustration of the effect of distribution of light is 
found in the case of the so-called changeable silks. 
Such fabrics have a nap, and when the fibers end in 
the direction toward the light the latter penetrates 
the fabric and is deeply colored by multiple selective 
reflections. The light that comes from other direc- 
tions is more or less specularly reflected, thus under- 
going less change by Selective absorption, with the 
result that various portions of the surface appear 
differently. Adding to the foregoing another property 
of aniline dyes and the colors of changeable silks are 
accounted for. For instance, a dye which in solu- 



170 COLOR AND ITS APPLICATIONS 

tion appears pink or purple in color is often found to 
reflect green light predominantly in the solid state. 
Thus the specularly reflected light in the case of the 
changeable silk is sometimes roughly complementary 
to the light that penetrates the fabric and is returned 
after multiple reflections which in effect correspond 
to traversing a certain depth of an aqueous solution 
of the dye. A color will often appear different by 
reflection than when examined * over-hand' by look- 
ing through the fibers by glancing along the surface 
at a grazing angle (#75). 

43. After-images, — It has been seen that the 
retinal excitation requires appreciable time to decay 
after the stimulus has been removed. If the filament 
of an incandescent lamp be viewed for an instant and 
the eye be then closed, an image brighter than the 
surroundings will persist for some time. This has 
been called a positive after-image. Soon, depending 
upon the intensity of the stimulus, the image will 
reach a stage of decay when it appears darker than 
the surroundings. If the closed eyelid be illumi- 
nated the visual field will appear brighter than in the 
case where the eyelid is shielded from the light by 
the hand placed gently against it. In the former 
case the after-image will remain * positive' a shorter 
time than in the case of the darker surroundings. 
The same will be found when viewing the after- 
image against various white or gray backgrounds 
with the eyelid open. The effect of the brightness 
of and exposure to the stimulus on the duration of 
the after-image is shown in Fig. 81. In this experi- 
ment the actual duration was somewhat longer than 
indicated, the criterion being the time aftlff exposure 
that was required for the after-image to decay to a 
certain definite, though faint, appearance- The after- 



EFFECT OF ENVIRONMENT ON COLORS 



171 



images were found to go through a certain cycle of 
brightness and hue changes which were not recorded. 
The stimulus was a bare tungsten filament varied in 
brightness by a variable sectored disk which was 
rotated at a high speed. The brightness is given in 
terms of candles per square inch. The after-images 
were observed against a faint background produced 
by the illumination of the closed eyelid by a small 
amount of stray light in the room. The changes in 

lOOt 

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C 70 

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EXPOSURE (SECONDS) 
Fig. 81. — Effect of brightness on the duration of the after-image. 

hue were not recorded. Positive after-images, ob- 
tained by fixating, for a few seconds, a white paper 
illuminated by sunlight, can be seen for a brief 
period, but they rapidly decay to a brightness lower 
than that of ordinary surroundings. In their decay 
they pass through a series of hues, namely blue- 
green, indigo, violet-pink, dark orange, etc., which 
are more or less definite. Helmholtz explains the 
colored after-images obtained in the above manner 
by assuming different rates of decay of the three 
hypothetical color sensations which are the basis of 



172 COLOR AND ITS APPLICATIONS 

the Young-Helmholtz theory of color vision (#47). 
The negative after-image is explained by Helmholtz 
as being caused by retinal fatigue, due to the origi- 
nal bright image of the white object. On stimu- 
lating the whole retina with white light the portion 
previously fatigued does not respond in the same 
degree as the unfatigued portions. It is difficult, 
however, to reconcile all the facts gleaned from 
studies of after-images with this fatigue hypothesis. 
Hering explains these phenomena by assuming that 
the retina is not fatigued, but that a metabolic change 
is aroused which is opposite in character to that pro- 
duced by the original excitation (# 49). After-images 
are also produced by fixating colored objects. For 
instance, if the object shown in Fig. 85 be fixated 
for a few seconds and the eye be turned toward a 
white surface, a pink after-image will be seen where 
the green had previously stimulated the retina. After- 
images viewed in this manner will usually appear 
approximately complementary in hue to the original 
stimulus. A striking illustration of approximately 
complementary after-images can be performed with 
the apparatus shown in Fig. 35. Here we have a 
large variety of colors, the corners of the triangle 
appearing red, green, and blue. After fixating the 
color triangle for a few seconds, if the lights be turned 
off and white light be permitted to illuminate the 
white opal-glass surface, approximately complemen- 
tary after-images are seen. The experiment is strik- 
ing, owing to the many colors present. In order to 
produce the complementary after-images in a striking 
manner, it is necessary to stimulate the retina with 
white light. If the experiment be performed in a 
dark room after the lights in the colored triangle are 
extinguished, the ordinary after-images will be per- 



EFFECT OF ENVIRONMENT ON COLORS 173 

ceived, depending upon the color of the stimulus, 
its intensity, and other factors. After-images play 
quite an important part in vision, especially in viewing 
paintings and many other colored objects. For in- 
stance, if a blue skyline be viewed in juxtaposition 
with a green landscape, the natural shifting of the 
eye, even when attempting moderately to gaze steadily 
at the picture, will cause a shifting of the image of 
this dividing line upon the retina, with the result that 
the pinkish after-image due to the green stimulus 
(and likewise that due to the blue stimulus) will, in 
shifting above and below the horizon line, produce 
a vivid effect. Such phenomena often greatly add to 
the *life' of a painting. After steadily fixating a 
colored object for some time the color appears to 
become less saturated, and often there is an apparent 
change in hue. If a small piece of black paper on 
a larger background of red be fixated for a few 
seconds and the black paper be suddenly removed 
without disturbing the fixation, in its place will be 
seen a red spot more luminous than the sur?:oundings 
and of a more saturated reddish appearance. These 
experiments can be successfully performed with the 
Zimmerman colored papers. 

Successive contrast further complicates the appear- 
ance of colors. After stimulating the retina with 
red light, if the eye be suddenly fixated upon a 
green color the latter will appear more intense or 
saturated in color for a moment than if the eye had 
not been previously stimulated by the red light. On 
alternating these colors by means of a rotating disk 
at a low speed very brilliant effects are seen. Such 
successive contrasts are of importance in the study 
and application of color science. For instance, in 
permitting the eye to rove over a painting or bril- 



174 COLOR AND ITS APPLICATIONS 

liantly colored rug the appearance of the various 
individual colors are influenced by the previous 
retinal stimulation. The phenomenon of after-images 
has been only briefly touched upon here. The many 
details connected with them serve to show how intri- 
cate is the reaction of the retina to light. 

44. Simultaneous Contrast. — A detailed exami- 
nation of the mutual effect of two visual excitations 
is difficult, although the fundamental principles are 




Fig. 82. — Showing the eflfect of simultaneous contrast. The V*s are of equal 

brightness. 

not difficult to demonstrate. On viewing a gray 
pattern on a black background it appears brighter than 
when viewed upon a light background. The illus- 
tration shown in Fig. 82 was originally made by cut- 
ting the two figures in the form of a V from the same 
gray paper. On placing them as shown — one on a 
black and one on a white ground — the one on the 
white ground appears darker than the other. The 
effect is so persistent that a much darker gray can 
be placed on the black background and yet it will 
appear brighter than the one on the white ground. 



EFFECT OF ENVIRONMENT ON COLORS 175 

In fact it is practically impossible to make both appear 
alike by decreasing the brightness of the gray V on 
the dark ground. If several gray papers of different 
shades be placed as shown in Fig. 83, the edge of a 
lighter gray strip that is adjacent to a darker one 
will appear brighter than the other edge of the lighter 
gray strip. Such a specimen can be obtained by 
juxtaposing gray papers of different shades or by 
exposing a photographic plate in a very weak light 
by pulling out the slide of the plate holder a half- 
inch at a time at regular intervals. A print from a 




Fig. 83. — Showing induction. Each band, though uniform in brightness, appears 
brighter at the right-hand edge. 

successful negative will afford an excellent specimen 
for showing this phenomenon of induction. On rotat- 
ing such a disk shown in 6, Fig. 30, this phenomenon 
of induction can be demonstrated before a large 
audience. A striking demonstration of brightness 
contrast can be performed by viewing a gray paper 
through a hole in a white unilluminated screen. It 
appears very bright in contrast to the dark surround- 
ings. However, on illuminating the white surround- 
ings it is possible to make the former bright spot 
appear very dark by contrast. The demonstrations 
of color contrast are very numerous. M. E. Chev- 
reul,2 who directed the dyeing laboratory of the 
famous Gobelins many years ago, carried out very 
extensive researches on the effect of simultaneous 
contrast of colors as used in the textile industry. The 



176 



COLOR AND ITS APPLICATIONS 



R 




G 




record of his experiments is monumentaL Many 
have investigated the problem with the view of throw- 
ing light on color-vision theories, but many of the 
details garnered by the vast number of investigators 
remain unsatisfactorily explained. 

The intensity of the 
contrast effect diminishes 
rapidly on passing away 
from the point of maximum 
contrast. In Fig. 84 when 
two colors such as red and 
green are juxtaposed they 
appear accentuated in satu- 
ration and deeper in hue. 
In the case of these two 

Fig. 84. — An arrangement for COlorS they appear tO mOVO 

showing the reduction in the further apart in hue. When 

contrast effect by separating the ^ 

two colored objects. the two colors are separated 

as shown above, the con- 
trast effect practically disappears. If a disk of green 
be placed on a larger disk of red the contrast is very 
effective but if the smaller disk 
is outlined by a black circle the 
effect is reduced. If a gray figure, 
as in Fig. 85, be placed upon 
a green background, the gray 
figure will appear of a pink 
hue. The contrast hue induced 
in this manner is approxi- 
mately, though in general not pig. gS.-An arrangement 

exactly, complementary to the for showing the effect of 

exciting color. If a sheet of 

thin white tissue paper be placed 

over the arrangement shown in Fig. 85, the hue induced 

in the gray paper will be considerably strengthened. 



Green 



G roLj 



simultaneous contrast and 
after-images. 



EFFECT OF ENVIRONMENT ON COLORS 177 

Colored shadows were noticed by such great 
colorists as Leonardo da Vinci. These are illus- 
trated by casting the shadow of a pencil on white 
paper by light entering a window; only black and 
white contrast is seen. However, if from another 
direction light from an incandescent lamp be per- 
mitted to fall on the paper, another shadow is pro- 
duced. If the two shadows are of approximately 
the same brightness, the contrast colors of the shad- 
ows are very striking. The white ground outside 
the shadows is receiving the mixed light from the 
two illuminants, while the shadow cast by daylight 
receives light from only the incandescent lamp and 
appears yellow. The other shadow receiving only 
daylight appears blue. Shadows in a landscape 
appear blue because they receive light from the sky, 
and they often appear more vivid owing to contrast. 
Hering devised a most striking demonstration of 
binocular contrast. Red and blue glasses were placed 
in front of the two eyes respectively. The glasses 
sloped away from the eyes from the nasal to the 
temporal side. This permitted a control of satura- 
tion by introducing a white image from the sides by 
reflection. A black stripe on a white ground is 
doubled by increasing or decreasing the ocular diver- 
gence. The observed ground appears spotted, alter- 
nately blue and red, and sometimes a purplish white, 
which is due to * retinal rivalry.' The stripe seen 
through the red glass appears green and through the 
blue glass appears yellow. 

Briicke, Helmholtz, and others contend that the 
contrast effects are not of a physiological nature, but 
rather * errors of judgment'; that is, through the in- 
fluence of an adjacent color our * standard white' 
undergoes a change which alters our judgment. In 



178 COLOR AND ITS APPLICATIONS 

other words they claim the effect is of a psychological 
nature. These arguments have been repeatedly 
attacked and not without a considerable degree of 
success by Hering and others. For example, Mayer » 
devised methods for showing contrast color phe- 
nomena on surfaces large enough so that the colors 
could be matched by means of rotating color disks 
and thus he obtained quantitative measurements. 
He found that the subjective contrast colors were 
perceptible when viewed through a small opening 
for exposures as short as 0.001 second. They were 
also perceptible with instantaneous illumination from 
an electric spark, the duration of illumination in 
this case being of the order of one ten-millionth of a 
second. He concluded from this experiment that 
fluctuation of judgment was an untenable hypothesis 
for explaining subjective color contrast owing to the 
extremely short period of time of exposure. 

On the other hand, Edridge-Green ^ contends 4hat 
all our estimations of color are only relative and formed 
in association with memory and the definite objec- 
tive light which falls upon the eye. In many of the 
most striking contrast experiments the color which 
causes the false interpretation is not perceived at 
all; for instance, if a sheet of pale green paper be 
taken for white, a piece of gray paper upon it appears 
rose colored, but appears colorless when it is recog- 
nized that the paper is pale green and not white.' 

Thus the controversy continues. Many contra- 
dictory experimental data and opinions could be cited. 
Contrast may be due to unconscious eye-movements, 
to incipient retinal fatigue, to fluctuation or error of 
judgment, or to some other cause. Nevertheless 
there is no agreement as to the true explanation at 
the present time. 



EFFECT OF ENVIRONMENT ON COLORS 



179 



In Plate III are provided a number of arrange- 
ments which show the effects of simultaneous con- 
trast — brightness and hue contrasts — and various 
mixtures of these. Some of these illustrate restful 
and 'lively' combinations of color. There will be 
found much of interest in this illustration upon care- 
fully observing the various combinations alone and 
in comparison with adjacent ones. The four smaller 
squares in each row are identical in hue and bright- 
ness, which can be readily proved by the use of a 





Fig. 86. — Illustrating irradiation. 



mask. If this plate be covered with a white tissue 
paper some of the color contrasts are very striking. 

45. Irradiation. — This name is applied to the 
phenomenon of apparent increase in size of objects 
as they are increased in brightness. For instance, 
the crescent of the new moon appears larger than 
the remainder of the disk. A filament of an incandes- 
cent lamp appears, to increase in diameter as its 
temperature is raised from a dull red to its normal 
operating temperature. This effect has been attrib- 
uted by some to a spreading of the retinal image on 
account of a stimulation of nerves outside its actual 
geometric boundary. Others attribute the effect to 



180 COLOR AND ITS APPLICATIONS 

the aberrations in the optical system of the eye. In 
Fig. 86 the inner white square appears larger than 
the inner black square under high illumination, yet 
both are identical in size. The phenomena of simul- 
taneous brightness contrast is also evident, the white 
square amid black surroundings appearing brighter 
than the larger white square. Such effects are also 
perceptible with colored objects, as will be seen in 
Plate III. 

REFERENCES 

1. Amer. Jour, of Psych. 13, p. 481. 

2. The Principles of Harmony and Contrast of Colours, 1860. 

3. Amer. Jour, of Sci., July, 1893. 

4. Proc. Roy. Soc. B, 1913, 86, p. 110. 

OTHER REFERENCES 

Tschermak, Ueber das Verhaltnis von Gegenfarbe, Kompen- 
sationsfarbe und Kontrastfarbe, Phliig. Arch. 1907, 117, p. 204. 

F. Klein, Nachbilder, Uebersicht und Nomenklatur, Englemann's 
Arch. f. Physiol. 1908, Sup. Bd. p. 219. 

G. J. Burch, Proc. Roy. Soc. 1900, 66, p. 204. 

An excellent bibliography of the work on simultaneous contrast 
is given by A. Tschermak, Ueber Kontrast und Irradiation, Ergeb- 
nisse d. Physiol. 1903, p. 726. 

General references are Helmholtz, Handbuch d. Physiol. Optik 
and NagePs Handbuch. 



CHAPTER VIII 
THEORIES OF COLOR VISION 

46. Recorded writings, centuries before the be- 
ginning of the Christian era, contain speculations on 
the visual process. Alcmaeon, Empedocles, Aristotle, 
Democritus, Anaxagoras, Plato, and Diogenes are 
among the early writers and philosophers who pre- 
sented views on the nature of light and colors and 
on the process of vision. However, their specula- 
tions — which can hardly be considered otherwise, 
owing to lack of experimental data — are of little 
value since the modern development of the sciences. 
Color vision is largely physiological and psychologi- 
cal. The process of vision involves the physical 
cause, the physiological retinal process, and the psy- 
chological elements in the experience of sensations. 
As the knowledge of the three sciences involved in 
the process of color vision developed, theories of 
color vision became more intricate. In fact the vari- 
ous theories which are given credence at the present 
time are found on strict analysis to include in vary- 
ing degrees the physiologic process of vision, color 
vision, and the nature of perception. A theory of 
color vision must include all the foregoing factors, 
yet the dominating influence of one of these is usually 
perceptible in a given theory. In this chapter it is 
proposed to set forth briefly the latest theories which 
pertain to the subject of color vision. 

47. Young-Helmholtz Theory. — Thomas Young 
is credited with the conception of the three-color 

181 



182 COLOR AND ITS APPLICATIONS 

theory, but it seriously lacked experimental founda- 
tion until after the epoch-making work of Helm- 
holtz/ and since that time it has become known as 
the Young-Helmholtz theory. The hypothesis is that 
color sensations depend upon the action of three 
independent physiological processes involving three 
substances or sets of nerves. This theory approaches 
the matter chiefly from the side of physics; that 
is, the facts of color-mixture are used in building up 
the theory. There is no anatomical evidence that the 
three substances or sets of nerves are present. The 
primary sensation curves shown in Fig. 53 were 
determined by Koenig by being built up from experi- 
mental data; these have been proposed as repre- 
senting the three independent processes. They are 
plotted so as to enclose equal areas on the assumption 
that the sensation of white results from the stimula- 
tion of equal amounts of the three primary sensations. 
It is noted that spectral hues involve more than one 
of the primary sensations. 

This theory explains the main facts of color vision, 
although many details uncovered by experimenters 
have iiot yet been reconciled with it to the entire 
satisfaction of many scientists. After-images are 
explained by assuming fatigue of one or more of the 
processes in varying degrees. For instance after 
fatiguing the eye to green light a white surface 
appears an unsaturated purple — pink. Many of 
the observed facts in the study of after-images are 
only approximately conciliable with this theory. The 
problem of simultaneous contrast offered no diffi- 
culties to Helmholtz, because he assumed that the 
phenomenon is the result of * false judgment.' While 
it may be purely psychological, it appears probable 
to some that it is actually physiological in nature. 



THEORIES OF COLOR VISION 183 

one part of the retina being influenced by stimulation 
of another region. Color-blindness is explained by 
assuming that one or more of the three processes are 
absent, the remaining process (or processes), if 
necessary, being assumed to be * redistributed' to 
some extent. This theory has some advantages in 
explaining the cases of red and of green blindness 
by assuming the absence of the corresponding process 
and if necessary a slight modification of the other 
two. It fails to explain total color-blindness, however. 
When it is attempted to reconcile this theory with 
all the observed facts, one finds a highly complex 
state of affairs. Such a discussion is outside the 
scope of this chapter, therefore only the main theories 
and facts will be presented. Extended discussions 
will be found in the treatises referred to at the end 
of this chapter. The Young-Helmholtz theory satis- 
factorily explains the observed facts of color-mixture, 
but the chief objection to the hypothesis as it exists 
at the present time, is that it fails to explain many 
other facts, such as those of contrast. 

48. 'Duplicity' Theory.— This theory, which at- 
tempts to differentiate colorless and color vision, is 
chiefly associated with the name of Von Kries. It 
is based upon anatomical evidence of the existence 
of * rods' and * cones' in the retina. The former are 
assumed to be responsible for achromatic sensations 
and the latter for both achromatic and chromatic sen- 
sations. The rod action is supposed to be largely 
responsible for light sensation at twilight illumina- 
tion and is in general more responsive to rays of 
shorter wave-length. The cones, however, are sup- 
posed only to act under stimuli of brightnesses repre- 
sented by the range above twilight illumination and 
not to be greatly increased in sensitiveness by dark 



184 COLOR AND ITS APPLICATIONS 

adaptation. Examination of the retina shows that 
the cones alone exist in the very center of the retina, 
the fovea centralis, and rods appear just outside of 
this and predominate in the outer zones. The chief 
observed facts that this theory explains fairly satis- 
factorily (perhaps because it was chiefly built up 
from these facts) are (1) colorless vision over the 
whole retina in dim light, for instance in moonlight, 

(2) the decreased sensitivity of the fovea in twilight, 

(3) the shift in the maximum of the luminosity curve 
of the eye (Purkinje effect) at low illumination, (4) 
the absence of such a shift for foveal vision, (5) no 
achromatic threshold is found for any light for foveal 
vision, (6) no achromatic threshold for red light for 
any region of the retina, and (7) colorless vision over 
the whole retina in the case of the totally color blind. 
Some of the experiments with color-blind eyes fur- 
ther support the theory. For instance the luminosity 
curve for a totally color-blind eye at ordinary illumi- 
nations is similar to that for a normal eye for twi- 
light vision. There are also evidences of diminished 
foveal sensibility, abnormally good vision in twi- 
light, and decreased ability to fixate small objects 
with color-blind eyes. Further support is found in 
the presence of rods almost exclusively in the retinae 
of such nocturnal animals as the owl and bat. The 
supporting evidence in general is represented by 
more dependable and convincing data than in the 
case of any theory of color- vision. The * duplicity 
theory' does not attempt to explain color- vision, but 
is of interest here because of the attempt to separate 
vision into chromatic and achromatic processes. 

49. The Hering Theory. — The principal foun- 
dation of this theory ^ consists of facts such as those 
of contrast, and the apparent simplicity of black, 



THEORIES OF COLOR VISION 186 

white, and yellow as well as red, green, and blue. 
Hering assumes there are six fundamental sensa- 
tions coupled in pairs, namely, white and black, red 
and green, yellow and blue. In order to account for 
these six fundamental sensations he assumes the 
presence somewhere in the retinocerebral apparatus 
of three distinct substances. Each substance is 
capable of building up (anabolism) or of breaking 
down (katabolism) under the influence of radiant 
energy or its effects. The building up of the black- 
white substance causes a sensation of blackness, and 
the breaking-down of this substance, a sensation of 
whiteness. Likewise anabolism of the red-green 
substance is connected with the sensation of green 
and katabolism with red sensation. Similarly, the 
building up of the third substance produces blue, 
and the breaking down is associated with yellow 
sensation. For example, red rays cause a breaking 
down of the red-green substance, with the result that 
red sensation is experienced. It is claimed by many 
that this theory has an advantage over the Young- 
Helmholtz theory, because it deals more directly with 
the sensations of color. The theory has many en- 
thusiastic supporters and is fully as favored in this 
respect as its most formidable rival. A favorite 
argument in support of it is the observed fact that 
yellow appears to be a primary color because there 
is no simultaneous suggestion of both red and green 
in a yellow made by mixing these two colors (Fig. 
17). Many observed facts concerning after-images 
agree with the theory. For example, if the eye be 
stimulated by blue rays, anabolism will take place in 
the yellow-blue substance and an accumulation of 
the substance results. If now yellow rays are per- 
mitted to stimulate the same area of retina, the break- 



186 COLOR AND ITS APPLICATIONS 

ing down of the yellow-blue substance proceeds at a 
greater rate and the sensation is greatly augmented. 
Conversely yellow decreases the amount of substance 
and increases the rate of anabolism under the sub- 
sequent stimulation of blue rays. Positive after- 
images are explained by assuming a continuation 
of the anabolic (or katabolic) change for a brief period 
owing to chemical inertia. All the general phenomena 
of after-images are explained satisfactorily, but as in 
the case of the Young-Helmholtz theory, details are 
troublesome. Some of the data on color-blindness 
readily support the theory, but the latter must be 
modified in order to explain other data. Bonders ^ 
and others conclude that the Young-Helmholtz and 
Hering theories, having been formulated from dif- 
ferent points of view, have arrived at different con- 
clusions and that both are in part correct. This is a 
rather safe conclusion, but nevertheless an important 
one, inasmuch as they are both thus stamped with 
the partial approval of scientists highly familiar with 
the subject. 

50. Ladd- Franklin Theory. — In this theory ^ the 
rods and cones are used. Colorless sensations white, 
gray, and black, are assumed to be caused by a primi- 
tive photo-chemical substance which is composed of 
many 'gray' molecules. These exist in their primi- 
tive state only in the rods, but upon dissociation 
they cause the colorless sensation. In the cones 
the gray molecules undergo development and for 
some reason only a portion of the molecule becomes 
dissociated by rays of a given wave-length or color. 
The evolution of the gray molecule is assumed to take 
place in three stages diagrammatically shown in Fig. 
87. In the first stage the gray molecule exists, but 
is so constructed that it is disintegrated by light of 



THEORIES OF COLOR VISION 



187 



all colors, thus producing a white or a gray sensation. 
In the second stage the molecule has become more 
complex and contains two groupings. The disso- 
ciation of one of the latter 







STAGE 1 



(z^^z^^ 



stage: 2 





(V^0— 



STAGE 3 



causes a yellow sensation 
and the other, blue. Their 
simultaneous dissociation 
causes a sensation of white 
or gray. Molecules are 
assumed to exist in this 
stage in the outer zone of 
the retina, where neither 
red nor green can be per- 
ceived as such. In the 
third stage the yellow 
grouping is divided into 
two new combinations, the 
dissociation of one giving 
rise to a red sensation, the 
other producing a green 
sensation. If the red and 
green are dissociated sim- 
ultaneously, yellow sen- 
sation results, while all 
three (red, green, and blue) together produce gray. 
There is much of interest in this theory, and it 
appears to explain many observed facts satisfactorily. 

51. Eldridge- Green Theory. — Boll discovered a 
substance diffused in the retina which has been 
named visual purple. This discovery gave rise to 
hopes that a photochemical theory of vision would 
explain the observed facts, inasmuch as the visual 
purple was found to be sensitive to light. However, 
after the elaborate work of Kiihne the visual purple 
lost much of its significance in this respect. If an 







Fig. 87. — The evolution of the Ladd- 
Franklin gray molecule. 



188 COLOR AND ITS APPLICATIONS 

eye which has been unexposed to light for some time 
be cut out in a room illuminated by means of a dim 
red light, on removing the retina it appears a purple 
color under ordinary light. The color fades rapidly 
on exposure to ordinary intensities of illumination, 
passing through red and orange to yellow, finally 
disappearing. The yellow appearance is supposed 
to be due to the formation of another pigment, the 
visual yellow. The appearance of the preceding 
stages is thought to be due to mixtures of the visual 
purple and visual yellow in various proportions. It 
apparently has been established that normally the 
visual purple is confined to the outer portions of the 
rods. It is extracted readily by a watery solution of 
bile salts. Spectroscopic examination of this solu- 
tion shows it to have a maximum absorption for 
yellow-green rays and a minimum for red rays and 
is bleached by the rays in about the proportion that 
it absorbs them. The visual purple is so sensitive 
to light that pictures of very bright objects have been 
seen in purple and white on retinae of the eyes of 
animals. Such experiments have been performed 
by exposing an eye extracted from an animal which 
has been kept in darkness for some time. 

Many attempts have been made to weave the 
visual purple into a theory of vision. Edridge-Green ^ 
has done so, as briefly outlined below. He assumes 
*that the cones of the retina are insensitive to light, 
but sensitive to the changes in the visual purple. 
Light falling upon the retina liberates the visual 
purple from the rods, and it is diffused into the fovea 
and other parts of the rod and cone layer of the 
retina. The decomposition of the visual purple by 
light chemically stimulates the ends of the cones 
(probably through the electricity which is produced) 



THEORIES OF COLOR VISION 189 

and a visual impulse is set up which is conveyed 
through the optic nerve to the brain.' He further 
assumes that 4he visual impulses caused by the 
different rays of light differ in character just as 
the rays of light differ in wave-length. Then in the 
impulse itself we have the physiological basis of the 
sensation of light, and in the quality of the impulse 
the physiological basis of the sensation of color.' 
He also assumes 4hat the quality of the impulse is 
perceived by a special perceptive center in the brain 
within the power of perceiving differences possessed 
by that center or portion of that center. According 
to this view the rods are not concerned with trans- 
mitting visual impulses, but only with the visual 
purple and its diffusion.! On this theory he attempts 
to explain all the observed facts encountered. He 
concludes the paper, from which the foregoing is 
quoted, by stating that *I am not aware of any fact 
which does not support the theory.' Needless to 
say, however, there are those who entertain a dif- 
ferent opinion on this last and other points. 



REFERENCES 

1. Handbuch der Physiologischen Optik. 

2. Grundziige der Lehre vom Lichtsinn, Leipzig, 1905, p. 41. 

3. Archif. f. Opth. 1881, 1, p. 55; 1884, I, p. 15. 

4. Zeit. f. Psych, u. Physiol, der Sinnesorgane, 1892. 

5. Lancet, Oct. 2, 1909. 

OTHER REFERENCES 

Ebbinghaus, Theorie des Farbensehens, Zeit. f. Psych, u. Physiol, 
der Sinnesorgane, 1893. 

A. Koenig, Ges. Abhandlungen, Leipzig, 1903. 

M. Greenwood, Jr., Physiology of the Special Senses. 

W. Nagel, Handbuch der Physiologie des Menschen, 1905. 



190 COLOR AND ITS APPLICATIONS 

W. Wundt, Grundzuge der Physiologischen Psychologie, Leipzig, 
1911. 

H. Aubert, Grundziige der Physiologischen Optik, Leipzig, 1876. 

Captain W. de W. Abney, Colour Vision, London, 1895. 

F. W. Edridge-Green, Colour Blindness and Colour Perception, 
London, 1909. 

W. Nicati, Physiologie Oculaire, Paris, 1909. 

J. H. Parsons, Colour Vision, New York, 1915. 



1 



CHAPTER IX 
COLOR PHOTOMETRY * 

52. The relation between radiation of various 
wave-lengths and luminous sensation has long been 
the subject of investigation; but, notwithstanding the 
extensive data obtained, there is no general agree- 
ment as to a method that yields correct results. 
Much of the early data is practically useless at the 
present time, owing to the lack of control of various 
influential factors, due to the absence of definite 
knowledge regarding their ability to influence the 
judgment of brightness. This data of course has 
served well in lighting the pathway of investigation. 

From foregoing chapters it has been seen that the 
size of the photometric field, owing to the variation 
of retinal sensibility to colored light, is of importance 
in color photometry. Due to the Purkinje phenom- 
enon the brightness at which measurements are made 
also affects the results. In this connection it should 
be noted that the brightness of the photometric field 
as seen by the eye is sometimes greatly reduced by 
absorption of light in the optical path and by a small 
ocular aperture or artificial pupil of the instrument. 
Other factors, such as the adaptation of the eye and 
the character of the surrounding field, are influential. 
Most important is the method, for no two methods 
yield exactly the same results. It is well to remem- 
ber that the brightness of a colored area is so influ- 
enced by its environment that its determination, in 
comparison with a standard in an isolated photometric 

191 



192 COLOR AND ITS APPLICATIONS 

field, is not in general a measure of its brightness 
as it appears in another environment. Therefore 
the photometry of colored surfaces yields measure- 
ments of the brightness in terms of the particular 
standard used and the results depend upon the hue, 
saturation, and brightness of the comparison field, 
the surroundings, the condition of the eye, and the 
photometric method used. 

53. Primary Methods of Photometry. — A method 
of photometry should ordinarily have for its object 
the measurement of the illuminating value of the 
illuminant with respect to its ability to make objects 
visible by reflected or transmitted light. The method 
of visual acuity has been proposed and used by some 
for the measurement of illumination. Obviously such 
a method determines the defining power of the illumi- 
nant or of the light reflected or transmitted by an 
object. The criterion of such a method is usually 
the discrimination of fine detail or the adjustment of 
the illumination so that the detail appears to be 
equally legible as compared with a standard. In 
general this method is quite insensitive, and the 
results are greatly dependent upon fatigue and the 
state of adaptation of the eye. However, there is 
another complication, that of the spectral character 
of light. The experiments of the author described 
in #37 showed that a reduction in the amount of 
illumination or brightness of the acuity test object, 
when accompanied by certain changes in the spectral 
character of the illuminant, sometimes results in an 
increase in visual acuity. From these experiments 
it is seen that the method of visual acuity cannot 
be depended upon to determine the relative illumi- 
nating values of illuminants or of lights altered in 
spectral character by reflection from colored surfaces. 



COLOR PHOTOMETRY 193 

The visual acuity method is valuable in many cases, 
but it must be understood that a large amount of our 
seeing does not include the perception of fine detail 
at the limits of discrimination, but only requires the 
recognition of relatively large surfaces through dif- 
ferences in color and brightness. Even reading under 
ordinary conditions does not involve visual acuity at 
the limit of discrimination, for the illumination is 
usually far above that necessary to distinguish the 
type, and it has been shown that in reading the eyes 
recognize characters in groups, travel by jumps, and 
come to rest only a few times during their progress 
across a page. 

At one time the critical frequency method was 
looked upon as a possible solution of the problem of 
color photometry. It was shown in #38 (Figs. 75 and 
76) that if a brightness be alternated against darkness 
there is a certain minimum frequency of alternation, 
called the critical frequency, at which flicker just 
disappears. In general, it has been found that the 
critical frequency varies directly as the logarithm 
of the illumination or brightness of the test surface. 
Thus plotting these two factors yields a straight line, 
the slope of which is different for lights of different 
colors. The slope of this straight line changes 
abruptly at a very low illumination (thought by some 
to be the point at which the cones just cease to be 
sensitive to light) for lights of all colors with the ex- 
ception of red. By this method two surfaces were 
assumed to be equally bright when their critical 
or vanishing-flicker frequencies were equal. The 
method has proved too insensitive for practical use 
and too susceptible to various physiological factors, 
such as fatigue and adaptation. 

The ordinary direct comparison or equality-of- 



194 COLOR AND ITS APPLICATIONS 

brightness method is claimed by many to involve the 
only true criterion for the measurement of brightness. 
Others claim that its shortcomings have disqualified 
it for use in the photometry of lights of different colors 
and have accepted the flicker method. The latter 
method, which is in high favor for color photometry, 
has not been proved to measure the true brightness 
of a colored surface — if there be such a thing. Nev- 
ertheless, the uncertainties in the measurements by 
the direct comparison method has brought this or- 
dinary method into disfavor with many photometri- 
cians for the photometry of lights differing in color. 
The flicker method involves the alternation of two 
brightnesses — the standard and the unknown. A 
match is made with this instrument by altering both 
the brightness of the field due to one of the sources 
and the frequency of alternation. The two bright- 
nesses are considered equal when the frequency of 
alternation is such that a slight change of either 
illumination produces a just perceptible flicker. When 
there is no color difference between the two bright- 
nesses being compared, the theoretical frequency 
should be zero, but owing to imperfections in the 
photometric apparatus this condition is never obtained. 
The flicker photometer is based upon the fact that 
color difference is eliminated by mixing the two 
brightnessies by persistence of vision, the color flicker 
apparently disappearing before the brightness flicker. 
Numerous instruments have been devised for this 
purpose, but all involve this fundamental principle. 
No extended comparative study of flicker photometers 
has been made, although it is possible that instru- 
ments differing in design might yield different results. 
For instance, in many instruments the stimulus 
changes abruptly from one color to another, but in 



COLOR PHOTOMETRY 



196 



some the stimuli dissolve into each other. Whether 
or not such instruments yield different results is a 
question to be solved by further investigation. 

To summarize, the methods of visual acuity and 
critical frequency are impracticable owing to their 



10 
















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0.50 0.52 054 0.56 05& 0.60 0.62 0.64 0.66 
Fig. 88. — The results of four methods of photometry (Ives). 

extreme insensitiveness and the influence of eye fa- 
tigue and adaptation. The influence of the spectral 
character of light further complicates the visual 
acuity method. The direct comparison method, though 
claimed by many to yield measurements of *true' 
brightness is unpopular, owing to the imcertainties 
in the measurements. The flicker method, hoTyever, 
owing to its elimination of color difference and high 



196 COLOR AND ITS APPLICATIONS 

sensibility, had won many ardent supporters even 
before extensive investigations of the method had 
been made. 

In Fig. 88 are shown data obtained by Ives ^ 
with the four methods. In each case the standard 
was the total light from a tungsten lamp. Spectral 
colors were compared with this white standard. 
Curve V was obtained by the visual acuity method; 
Dy by the direct comparison; F, by the flicker; and 
C, by the critical frequency method. If the four 
methods gave identical results, the curves would coin- 
cide. The general shapes and positions of the max- 
ima are similar, but the areas under the curves are 
very different. The enormous area under curve V 
is in accord with the previous work of BelP and of 
Luckiesh,^ which showed that acuity was much 
better in monochromatic light than in light of ex- 
tended spectral character. 

54. Secondary Methods of Color Photometry. — 
Various schemes have been proposed and developed 
for eliminating color difference in heterochromatic 
photometry, such as the use of colored filters, and 
physical and chemical photometers used with filters 
that properly weigh the energy of various wave- 
lengths according to their light-producing effects. 
Among the latter possibilities are the radiometer, 
thermopile, selenium cell, photo-electric cell, and 
photographic plate. Obviously, in order to reduce 
measurements to absolute values, the transmission 
coefficients of the colored filters must be determined 
by some acceptable method. Likewise, determina- 
tions of the relation between radiation of various 
wave-lengths and the corresponding luminous sensa- 
tions and of the sensibility of the instruments to 
energy of various wave-lengths must be made before 



COLOR PHOTOMETRY 197 

the results obtained with the selenium cell, the radiom- 
eter, filters, etc., are useful in measuring brightness. 

Crova ^ suggested as a method of comparing lights 
possessing continuous spectra, but differing in color, 
the determination of their intensities at one wave- 
length, 0.582/z. The lights to be compared in this 
manner must not differ much in spectral energy dis- 
tribution from the black body. Rayleigh,^ Nernst,^ 
Fery and Cheneveau,^ Lucas,^ Rasch,^ and others 
have made various applications and modification of 
Crova's original proposal. The filter used by Crova 
consisted of an aqueous solution of anhydrous ferric 
chloride (22.321 grams) and crystallized nickelous 
chloride (27.191 grams), the total volume being 100 
c.c. at 15° C. A thickness of 7 mm. of this solu- 
tion was used which transmits energy from 0.63^ to 
0.534a( with a maximum of transmission at 0.582^, the 
wave-length which Crova found to be satisfactory for 
carrying out his proposed scheme. Ives ^° tested 
Crova's method by comparing the luminous intensities 
of a tantalum and a carbon incandescent lamp ^t 
various wave-lengths. He found that the wave-length 
for such a comparison lies between 0.56/x and O.SSju, 
depending on the range of temperature. The latter 
wave-length was found to hold best of all within the 
limits of temperature represented by ordinary incan- 
descent lamps of that time. Twelve years ago 
Fabry ^^ recommended the use of two or more col- 
ored solutions for eliminating color difference, having 
fijrst calibrated these solutions for thickness and trans- 
mission by an acceptable method. Aniline dyes were 
not used, because of the need for definite and re- 
producible solutions. By using two solutions, A and 
5, he was able to match the Carcel lamp with almost 
any illuminant. The solutions were made as follows: 



198 COLOR AND ITS APPLICATIONS 

A. Crystallized copper sulphate 1 gram 

Commercial ammonia (density 0.93) 100 c. c» 

Water sufficient to make one liter. 

B. Potassium iodide 3 grams 

Iodine 1 gram 

Water sufficient to make one liter. 

Ives and Kingsbury ^^ have recently investigated 
the problem of obtaining suitable solutions that would 
eliminate color difference after the manner pro- 
posed by Fabry. They developed a yellow solution 
containing 100 grams of cobalt ammonium sulphate, 
0.733 grams of potassium dichromate, 10 c.c. of 
1.05 sp. gr. nitric acid, and distilled water to make 
one liter at 20 deg. centigrade. The method of 
preparation is considered very important and is pre- 
sented in detail in the original paper. Of course a 
given depth or concentration of the solution has a 
different transmission for illuminants of different 
spectral character. The transmission values were 
determined by means of a flicker photometer by 
averaging the results obtained by specially selected 
observers. The transmission of the solution was 
found to vary considerably for different temperatures 
and the character and cleanliness of the glass sides 
of the containing cell were found to be of consid- 
erable importance. It was found possible to eliminate 
color difference in comparing many illuminants with 
the carbon lamp standard by placing the solution on 
either one side or the other of the photometer. 

Many have used aniline dyes and colored glasses. 
In practical photometry the use of colored glasses 
appears to be satisfactory for a large amount of work. 
The carbon lamp operating at about 4 w.p.m.h.c. is 
the present standard of luminous intensity. Properly 



COLOR PHOTOMETRY 199 

tinted bluish glasses used with this standard will 
eliminate the color difference when comparing tung- 
sten lamps with it. The transmissions of the tinted 
glasses can be obtained by averaging the determina- 
tions of a large number of observers, using the direct 
comparison method. Such a procedure is being used 
successfully in several laboratories for the above 
work where the color difference is not excessive. 
However, it is not a solution of the general problem 
of color photometry. 

Houston 13 in 1911 proposed the use of a filter 
composed of two solutions — copper sulphate and 
potassium dichromate — for closely approximating in 
transmission the luminosity curve of the eye, this 
filter to be used with an energy-measuring instru- 
ment. Koenig's visibility data were used as a basis 
for developing the solution. A proper solution would 
transmit rays of various wave-lengths in the propor- 
tions corresponding to the relative light-producing 
values of the various rays. It is necessary to cut 
off both the infra-red and ultra-violet rays and to re- 
duce the visible rays in just the correct relative pro- 
portions so that an energy-measuring instrument 
(bolometer, thermopile, or radiometer) will record 
data proportional to the luminous intensity. A dis- 
advantage of such instruments is found in their 
extreme sensitiveness to outside disturbances. For 
instance, the galvanometer used in the procedure 
must be of a liigh order of sensibility and therefore 
must be set up where it will be free from mechanical 
and magnetic disturbances. Karrer,!^ recently follow- 
ing Houston's lead, similarly employed the visibility 
data obtained by Ives. By using three solutions he 
was able to produce a screen whose transmission 
curve closely approached this luminosity curve of the 



200 



COLOR AND ITS APPLICATIONS 



eye. The solutions were made by dissolving (1) 
57.519 grams of cupric chloride, (2) 1.219 grams of 
potassium bichromate, and (3) 9.220 grams of ferric 
chloride, each in one liter of water. A triple cell 
was used, each compartment being 1 cm. thick. 

The selenium cell has been used for stellar pho- 
tometry and for other special work, owing to its 
change in resistence on being illuminated. However, 
it has not yet found a place in color photometry, be- 
cause at present it is too erratic and undependable. 





1 1 






6elehium I 




BtptoElecfric 






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K 


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040 0.46- 0.50 0.55 0.60 0.65 070 0.75 

Fig. 89. — spectral sensibilities of selenium and photo-electric cells compared 
with the spectral sensibility of the eye. 

Its sensibility to energy of various wave-lengths ap- 
pears to depend upon the method of making the cell, 
and is in general far different from that of the eye. 
A sensibility curve is shown in Fig. 89, compared 
with the luminosity curve of the eye. The maximum 
change in resistance is usually due to energy of the 
longer visible wave-lengths. Obviously a filter that 
properly weighs the energy of various wave-lengths 
according to its light value and to the spectral sen- 
sibility of the cell, must be used for the photometry 
of illuminants of extended spectral character. 

The photo-electric cell has been used in special 
cases of scientific investigation for detecting the 



COLOR PHOTOMETRY 201 

presence of radiant energy. Surfaces of potassium, 
zinc, and other elements and compounds in vacuo 
exhibit the property of emitting electrons when illumi- 
nated. The maximum effect is usually found in the 
short-wave visible region, as illustrated by a sensi- 
bility curve of a photo-electric cell, shown in Fig. 89. 
As in the case of the selenium cell, the photo-electric 
cell is too erratic at the present time to be adopted 
as a means of photometering lights of different colors. 
The strengths of the electronic currents measured 
by means of a sensitive electrometer or galvanometer 
afford a measure of the relative intensities of the 
illumination of a given spectral character when the 
characteristics of the cell are shown; that is, when 
the relation between the intensity of illumination and 
the photo-electric effect is known. Lights differing 
in spectral character cannot be compared by means 
of the photo-electric cell unless a correcting filter is 
used after the manner necessary with the selenium 
cell. 

The photographic plate affords another possible 
method for the photometry of lights of different color, 
but its general adoption is discouraged, owing to lack 
of uniformity of the emulsion both as to thickness 
and sensibility. Some of the difficulty could be 
obviated by using plates made of plate glass. The 
panchromatic plates must be used, because the ordi- 
nary plate is not appreciably sensitive to rays of 
longer wave-length than 0.48/x, the maximum of 
sensibility being in the extreme violet region of the 
spectrum. The relative sensibility of a certain com- 
mercial panchromatic plate, for equal amounts of 
energy of various wave-lengths, is shown in Fig. 90 
compared with the spectral sensibility of the eye. 
In order to make the plate record the values of col- 



202 



COLOR AND ITS APPLICATIONS 



ored brightnesses as determined with a flicker pho- 
tometer, an accurate filter was made which consisted 
of aesculine, tartrazine, rhodamine, naphthal green, 
and glass three-eighths of an inch thick. How nearly 
this filter performs its intended purpose is shown in 
Fig. 91 by the circles in comparison with the lumi- 
nosity curve of the eye which is represented by the 
full line curve. This filter was used with the pan- 



100 
90 
80 
^ 70 
S 60 
§50 
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%A0 044 



045 Q52 056 060 0.64 0.66 

UC, WAVE LEMGTH 



Fig. 90. — spectral sensibility of a panchromatic photographic plate. 

chromatic plate considered above, for which it was 
made by Ives and Luckiesh^^ for various photometric 
problems. In using the photographic plate for pho- 
tometric purposes it must be remembered that, in 
general, the product of intensity of illumination and 
time of exposure is not a constant for equal photo- 
graphic effect. The relation between exposure and 
intensity of illumination for a constant photographic 
effect as discovered by Schwartzchild is IP = i T^ 
where / and i are the larger and smaller intensities 
and T and t are the larger and smaller periods of 



COLOR PHOTOMETRY 



203 



exposure. The value of p varies with different plates, 
generally lying between 0.75 and unity. The manner 
of development, the temperature, and other obvious 
factors influence the results so that the photographic 
method becomes unattractive except for special prob- 
lems. As already stated, the use of these so-called 
physical or chemical photometers, while obviating 
color difference in practise, does not preclude the 
necessity of establishing the relation between lumi- 
nous sensation and radiation of various wave-lengths 
by an acceptable method of color photometry. 



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0.40 0.44 048 



0.72 



Fig. 91. 



0.52 056 0.60 0.64 0.68 
An accurate color filter for the panchromatic plate considered in Fig. 90. 



55. Direct Comparison and Flicker Methods. — 
Only two primary methods for the photometry of 
lights differing in color are worthy of consideration, 
namely the direct comparison and flicker methods, 
the other two being ruled out of consideration for 
reasons already given. These two methods have 
been compared by many observers, but much of the 
work is so incomplete that it yields little data for a 
thorough comparison. It is desirable that the method 
finally acceptable for photometering lights of different 
colors should measure light-value with the same 
order of definiteness as other physical measurements 
are obtained. 

Dow 16 compared these two methods by using 



204 COLOR AND ITS APPLICATIONS 

colored lights produced by means of red and green 
glasses at different intensities and with different 
field sizes. He found with the direct comparison 
method that the ratio of the red to the green bright- 
ness decreased with decreasing illumination, the 
decrease being rapid below 0.3 meter candle, — the 
well-known Purkinje phenomenon. With the flicker 
method this decrease was slight. With a small pho- 
tometer field the change in the ratio of red to green 
was considerably less. In general Dow's results 
indicate that the flicker method is less influenced by 
the size of the field or by a change in the illumina- 
tion than the direct comparison method. 

P. S. Millar ^^ compared mercury vapor arcs with 
incandescent lamps over a wide range of illumina- 
tions. The Purkinje phenomenon was in evidence 
in the direct comparison measurements but absent 
in the results obtained with a flicker photometer. 
In other words, with the former method the apparent 
brightness of the side of the photometer field illumi- 
nated by light from the mercury arc did not decrease 
as rapidly as the brightness of the other side illumi- 
nated by light from an incandescent lamp, as the 
illumination decreased. Stuhr ^^ compared the four 
methods — namely, visual acuity, critical frequency, 
direct comparison, and flicker. He found the critical 
frequency and flicker methods to yield identical 
results, but these differed from the results by the 
other two methods. Various physiological factors, such 
as field size and illumination, were not considered. 

Ives ^^ carried out an extensive series of investi- 
gations which represent the most elaborate and 
thorough work yet done on the problem. He con- 
cluded that the flicker method is more sensitive than 
the direct comparison method and that the results 



COLOR PHOTOMETRY 205 

are more reproducible. He discovered that the flicker 
method exhibited a * reversed Purkinje effect' and 
found, as other investigators had, that the two 
methods yielded different results in general, but con- 
cludes that the flicker method yields, under certain 
specified conditions, a measure of true brightness. 
Much evidence obtained throughout these investiga- 
tions and some obtained by the author and others 
point favorably to the flicker method as the best 
method of photometry. However, notwithstanding 
the extensive investigations, some take the stand that 
the case has neither been decided against the direct 
comparison method nor in favor of the flicker method. 
This conclusion is perhaps justifiable. However, 
considering the unsatisfactoriness of the former 
method, there is considerable virtue in the adoption 
of the latter method with its many satisfactory fea- 
tures in default of a method which has been definitely 
proved to yield the desired measurements. Ives in 
his early papers did not emphasize the differences 
in the results obtained by the two methods. His 
results were plotted in the form of luminosity curves 
of the eye, so that without careful inspection, the 
results by the two methods, under certain conditions 
of high illumination and small field size, do not 
appear to differ greatly. In order to determine the 
magnitude of these outstanding differences the au- 
thor 20 carried out an investigation, a portion of the 
results {L) being plotted in Fig. 92. Red and blue- 
green lights were used. The ratio of the intensity 
of the red to that of the blue-green light is plotted 
for a wide range of illuminations. The illumination 
values are those obtained with the flicker photometer 
and a standard tungsten lamp, but are not corrected 
for the absorption of the photometer, which, owing to 



206 



COLOR AND ITS APPLICATIONS 



a complex optical path, was considerable, or for 
reduction due to the small artificial pupil. It is seen 
that the flicker method exhibits a reversed Purkinje 
effect and the direct comparison method the true 
Purkinje effect, and further that the ratio of the 
red to the blue-green brightness obtained by the 
direct comparison method is only about 62 per 
cent of that obtained by the flicker method for 



r.5 

1.4 h 



ao 



1.3 
1.2 
I.I 
1.0 
0.9 
0.8 
0.7 
0.6 
0.5 
OA 
0.3 
0,2 
0.1 
0, 



^^=; 



5^ 



~iz: 



(Flick er) Observer K 



(Flicker') Observer L 



o,tQUALiTY Of Brightness) Observer 



I 2 3 4 5 6 7 8 9 10 II 12 15 14 15 
METERCAttDLESCOn PHOTOMETER SCREEM) 

Fig. 92. — Resixlts by flicker and direct comparison photometers, illustrating 
differences including the Purkinje effect and a reversed effect. 



a large range of illuminations. The results as to the 
reversed Purkinje effect were verified in general by 
another observer {K). It is seen that he did not 
obtain the same results as the author, even with 
the flicker photometer, by about 13 per cent. A 
similar difference, though in general not as great, is 
found in Ives' data when the lights of the correspond- 
ing dominant hues (0.64/x and 0.52^) are compared, 
even though in his measurements the spectral colors 
were always balanced against a white light. The 
same extreme difference in the results by the two 



COLOR PHOTOMETRY 207 

methods was confirmed by the writer using the same 
glasses a year later. The field size in the foregoing 
experiments was rather large — about ten degrees — 
but a large difference persists even with smaller 
fields, though not to such an extent. 

Morris- Airey ^i suggested that the differences 
between the two methods might be due to the dif- 
ferent rates of rise of the sensation with different 
colors. The author ^2 studied this factor and showed 
that the maximum of a flickering red light was con- 
siderably greater than that of the blue-green light 
for a large range of flicker frequencies when the 
brightnesses of the two lights were those obtained 
by a direct comparison balance. This in itself did 
not prove which, if either, is the correct method. 
However, another experiment was performed which 
is perhaps as convincing as any yet performed in 
indicating that the flicker photometer when properly 
used is inappreciably influenced by the different 
rates of growth and decay of color sensations. Lights 
differing greatly in spectral character, but alike in hue, 
were compared by the two methods and identical 
results were obtained. Two yellow lights were ob- 
tained by means of filters of aqueous solutions of 
potassium dichromate used before a tungsten lamp. 
In one of the solutions was dissolved some neodym- 
ium ammonium nitrate, which absorbed all the spec- 
tral yellow. The two lights now nearly matched in 
hue and were readily brought to an exact c6lor-match 
by altering the concentration and by adding a little 
ordinary yellow or orange dye to one solution. The 
spectral characters of these two lights are shown in 
c and dj Fig. 17. Two 'white' lights were also com- 
pared, one consisting of the total light from a tung- 
sten lamp, the other being made up of narrow regions 



208 COLOR AND ITS APPLICATIONS 

of the spectrum, respectively in the red and blue- 
green. In both cases no difference in the ratio of 
the intensities of the two lights of the same color 
was detected in the results by the two methods, 
the accuracy being well within one per cent. It was 
also shown that red and blue-green lights add, 
whether by direct superposition or by alternately 
flickering them as in the flicker photometer, when 
the flicker is not more than barely apparent. 

Further investigations may show that the flicker 
photometer is influenced by the different rates of 
growth and decay of color sensations, but the fore- 
going experiments indicate that such influence is 
slight. Ferree 23 has attacked this problem and has 
reported some interesting preliminary results. The 
flicker method possesses many desirable character- 
istics, yet at present it can hardly be accepted as 
yielding *true' measurements of brightness unless 
the difference in the results, obtained by this method 
and by the direct comparison method, be ignored. 
Where color differences are large — just where such 
a method as the flicker method is most desired — the 
results by the two methods vary most widely. 

56. Luminosity Curve of the Eye. — Ives found 
that the spectral luminosity curve obtained with the di- 
rect comparison photometer by the * cascade' method 
(involving small steps of slight hue difference) agrees 
at high illumination for a small field with the 
curve obtained by the flicker photometer. He also 
found that the latter method fulfilled certain funda- 
mental axioms, namely, that the sum of several indi- 
vidual brightnesses of different hue must equal the 
brightness of the whole and that if each of two bright- 
nesses of different hue equal a third brightness, they 
must be equal to each other, while the direct com- 



COLOR PHOTOMETRY 



209 



parison method did not. While these experiments 
point with favor to the flicker method, it is true that 
a method can fulfill these requirements and yet not 
yield measurements of *true' brightness. However, 
it appears at the present time that the balance of 
experimental data is strongly in favor of the flicker 
photometer. For this reason the relation between 
radiation of various wave-lengths and their physi- 
ologic effect in producing luminous sensation as 



1.0 
0.9 



08 



5 0.3 

LiJ 



O.I 













































4 


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1 

i 


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k 


















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/ 


/ 














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/ 


J 


Y 
















^ 










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*^ 


















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k 





040 



0.44 



048 



0.52 036 0.60 

Jl, WAVELENGTH 



0.64 



O.60 



Fig. 93. — VisibiUty data. (See Table XVI.) 



obtained with the flicker photometer is of consid- 
erable interest. Ives ^^ determined the luminosity 
curves of 18 observers which he has published in 
comparison with the mean curve. Later he deter- 
mined the luminosity curves for 25 more observers, 
which mean, he states, agrees well with that of the 
previous eighteen. Nutting ^^ has recently obtained 
such data with 21 observers. The apparatuses used 
by both Nutting and Ives were practically the same 
as shown in Fig, 51, the source W being eliminated. 



210 



COLOR AND ITS APPLICATIONS 



The data as presented by Nutting are shown in Fig. 
93 and Table XVI compared with Koenig's^e origi- 

TABLE XVI 

The VisibiUty of Radiation (See #92) 



Wave 
length (/x) 


Nutting 

mean 

visibility 


Ives mean 


Koenig mean 


Computed 

from Nutting*s 

formula 


0.400 


0.002 








0.410 


0.003 








0.420 


0.008 








0.430 


0.012 








0.440 


0.023 


0.029 1 







0.450 


0.038 


0.047 1 


0.158 




0.460 


0.066 


0.073 1 


0.201 




0.470 


0.105 


0.107 1 


0.250 





0.480 


0.157 


0.154 


0.302 


0.135 


0.490 


0.227 


0.236 


0.370 


0.232 


0.600 


0.330 


0.363 


0.476 


0.358 


0.510 


0.477 


0.696 


0.670 


0.614 


0.520 


0.671 


0.794 


0.830 


0.675 


0.530 


0.835 


0.912 


0.960 


0.824 


0.540 


0.944 


0.977 


0.996 


0.933 


0.550 


0.995 


1.000 


0.990 


0.994 


0.560 


0.993 


0.990 


0.945 


0.993 


0.570 


0.944 


0.948 


0.875 


0.939 


0.580 


0.851 


0.875 


0.780 


0.839 


0.590 


0.735 


0.763 


0.680 


0.717 


0.600 


0.605 


0.635 


0.585 


0.586 


0.610 


0.468 


0.509 


0.492 


0.456 


0.620 


0.342 


0.387 


0.396 


0.343 


0.630 


0.247 


0.272 


0.300 


0.236 


0.640 


0.151 


0.176 


0.210 


0.158 


0.650 


0.094 


0.104 


0.128 


0.108 


0.660 


0.051 


0.068 1 


0.070 


0.072 


0.670 


0.028 


0.044 1 


0.032 




0.680 


0.012 


0.026 1 






0.690 


0.007 








0.700 


0.002 










1 Extrapolated. 

nal data which was obtained by the direct comparison 
method. Nutting has extended the observations well 
into the red and violet regions of the spectrum by 



COLOR PHOTOMETRY 211 

using sources emitting line spectra. Koenig's data 
are shown in curve Ky Ives' data in curve /, and 
Nutting's data in curve N. Nutting developed a 
formula of the form V= V^R^ e^^"~^^ from which 
the values given in Table XVI and represented in Fig. 
91 by the circles have been computed. F^ in the 
formula represents the maximum light-producing ef- 
fect, R = ^, a = 181, and V the visibility or rela- 
tive light-producing value of energy of any wave- 
length, X. The maximum sensibility is at X^ax = 
0.555m. The computed values are found to coincide 
practically with Nutting's mean luminosity curve be- 
tween wave-lengths, 0A8fjL and 0.65^. 

REFERENCES 

1. Phil. Mag. 1912, 24, p. 847. 

2. Elec. World, 1911, 58, p. 637. 

3. Elec. World, 1911, 58, p. 450, p. 1252. 

4. Comp. Rend. 93, p. 512. 

5. Phil. Mag. June, 1885. 

6. Phys. Zeit. 1906, 7, p. 380. 

7. Bui. Soc. Inst. Elec. 1909, p. 655. 

8. Phys. Zeit. 1906, 6, p. 19. 

9. Ann. d. Phys. 1904, 14, p. 193. 

10. Phys. Rev. 1911, 32, p. 316. 

11. Comp. Rend. Nov. 1913; Trans. I. E. S. 1913, 8, p. 302. 

12. Trans. I. E. S. 1914, 9, p. 795. 

13. Proc. Roy. Soc. A, 1911, p. 275. 

14. Lighting Jour. Feb. 1915; Phys. Rev. 1915, 5, p. 189. 

15. Trans. I. E. S. 1912, p. 90; Elec. World, 1912, 60, p. 153. 

16. Phil. Mag. 1910, 19, p. 58. 

17. Trans. I. E. S. 1909, 4, p. 769. 

18. Kiel, Phil. Diss. Vol. 19, 1908, p. 50. 

19. Phil. Mag. 1912, 24, p. 149, p. 170. 

20. Elec. World, Mar. 1913, p. 620. 

21. Electrician (Lon.), Aug. 20, 1909, p. 758. 

22. Phys. Rev. N. S. 1914, 4, p. 11. 



212 COLOR AND ITS APPLICATIONS 

23. Before. I. E. S. 1914. 

24. PhiL Mag. 1912, 24, p. 853. 

25. Trans. I. E. S. 1914, 9, p. 633. 

26. Ges. Abhandlimgen. 

OTHER REFERENCES 

On the Photo-electric Cell: 

H. Dember, Beiblatter, 1913, No. 16; p. 1044. 

Nichols and Merritt, Phys. Rev. 1912, 34, p. 475. 

F. K. Richtmeyer, Trans. I. E. S. 1913, p. 459; Phys. Rev. July, 

1915. 
H. E. Ives, Phys. Rev. N. S. 1914, 3, p. 68, p. 396. 

On the Selenium Cell : 

Seig and Brown, Phys. Rev. N. S. 1914, 4, p. 48, p. 85; 5, p. 65, 

p. 167. 
F. Townsend, Sci. Abs. A, 7, 2869. 
A. H. Pfund, Phys. Rev. 1912, 34, p. 370; Light. Jour. 

1913, p. 128. 
T. Torda, Electrician, 1906, 56, p. 1042; Sci. Abs. 9, 771. 
Joel Stebbins, Astrophys. Jour. 1908, 27, p. 183. 

On Color Photometry: 

E. P. Hyde and W. E. Forsythe, The Visibility of the Red End 
of the Spectrum, Phys. Rev. July, 1915; Astrophys. Jour. Sept. 
1915. 

Irwin J. Priest, A Proposed Method etc., Phys. Rev. July, 1915. 

E. F. Kingsbury, A Flicker Photometer Attachment for a 
Lummer-Brodhun Photometer, Jour. Frank. Inst. Aug. 1915, 
p. 215. 

H. E. Ives and E. F. Kingsbury, Flicker Photometer Measure- 
ments on a Monochromatic Green Solution, Phys. Rev. 1915, 
5, p. 230. 



CHAPTER X 

COLOR PHOTOGRAPHY 

57. At the present time no processes of color 
photography have been developed which employ the 
simple principle of fixing the colors of Nature directly 
upon the photographic plate by chemical means. O. 
Wiener \ discusses the use of body colors which 
would assume the colors corresponding to the rays 
of light by chemical modification. Carey Lea^ ob- 
tained a form of silver photochloride which assumed 
different colors on exposure to various rays, but no 
means was found for fixing them. Most of the com- 
mercial methods employ colored media which repro- 
duce colors by one of the common methods of color- 
mixture. In the first place the emulsion must be 
sensitive to all visible rays, and preferably the plate 
should be sensitive to light rays, in closely the same 
manner as the eye. There are no commercial plates 
endowed with the latter characteristic, so panchro- 
matic plates are usually used with an approximate 
color filter. Ray filters of the accuracy approaching 
that illustrated in Fig. 91 are rare, but for accurately 
photographing colored objects in their true values of 
light and shade, carefully made filters must be used 
with panchromatic plates, because the latter differ 
greatly in spectral sensibility from the eye (Fig. 90). 
In other words, a plate must be rendered of the same 
relative sensibility to the various visible rays as the 
eye by the use of sensitizing dyes and ray filters. 

213 



214 COLOR AND ITS APPLICATIONS 

Fortunately ordinary photography does not require 
such a high degree of accuracy. 

About a century ago Seebeck discovered that 
silver chloride becomes tinted by exposure to light 
with an accompanying chemical action. It is also 
possible by properly selecting luminescent salts to 
produce a series of tints after exposure which are 
very effective. Such colors cannot be fixed, and 
therefore are of little practical interest. The devel- 
opment of color photography has been confined 
largely to two methods. In one the phenomenon of 
interference of light waves is utilized to reproduce 
colors directly, while the other method is based upon 
the principles of color-mixture — both additive and 
subtractive (#18, #19). In the latter method artificial 
color-screens are used. Sometimes these are of 
minute size, as will be shown later. 

58. Lippmann Process. — The method employ- 
ing interference of light waves is originally due to 
Becquerel,^ but Lippmann's name is usually asso- 
ciated with the process, owing to the improvements 
which he devised after extensive investigation. ^ 
Zenker ^ in 1868 explained the colors sometimes ex- 
hibited by spectrograms made on silver chloride 
plates as due to the interference of light waves 
reflected from layers of metallic silver which are 
originally produced by stationary light waves. Among 
those who have investigated the process are Wiener,^ 
Neuhaus,^ Valenta,^ Lehmann,^ and Ives.^'^ 

In the Lippmann process the sensitive film is 
backed by a film of clean mercury which acts as a 
reflector. As light which has passed through the 
thin film strikes the layer of mercury it is reflected 
back on its path, and owing to the disappearance of 
energy at certain points through interference, the 



Red 



COLOR PHOTOGRAPHY 215 

silver compound is acted upon only in layers — at 

the antinodes. The phenomenon is diagrammatically 

shown in Fig. 94. The silver compound, instead of 

being acted upon throughout the 

thickness of the film, is largely 

reduced in thin laminae the dis- v^ioooooox ^re-g-/? 

tance between which is one-half a |^|oooooooo<^/y5> 

wave-length of the light producing 

them. EspeciaUy fine-grain plates "itl^ii^rre^pfo! 

must be used in order to produce duced in the Lippmann 

the very minute structure. The p^®^®^^- 
emulsion must be sensitive to all colors which 
are so made by the use of certain sensitizers. 
This discovery is due to H. W. Vogel, in 1873, who 
found that silver bromide by treatment with certain 
sensitizing dyes, such as eosine and cyanine, was 
rendered sensitive to rays of longer wave-length than 
when untreated. 

If the plate after exposure in the Lippmann process 
be developed and illuminated by white light, from 
various parts of the film only colored light escapes to 
the eye and a photograph in colors is seen. It is 
easy to account for the reproduction of pure spectral 
colors, but the general theory has been the subject 
of much discussion too extensive to dwell upon here. 

59. Wood Diffraction Process. — This method, 
invented by R. W. Wood in 1899, depends upon the 
phenomenon of interference, though in a different 
manner. It depends upon the principle that all colors 
may be matched in hue by mixtures of three primary 
colors, red, green, and blue, each consisting of a 
narrow band of the visible spectrum. These spectral 
primaries lie near the regions of the spectrum cor- 
responding respectively to 0.65m, 0.52/z, and 0.45/1. 
This process utilizes diffraction gratings for the pro- 



216 



COLOR AND ITS APPLICATIONS 



duction of the primary colors. If a point source of 
light or an illuminated slit be viewed through a dif- 
fraction grating (#9), not only will an image of the 
source be seen, but displaced on either side a series 
of spectra will be seen. The displacement of the 
spectra from the line joining the eye and light source 
will depend upon the number of lines per inch in the 
grating; the fewer lines per inch the less is the dis- 
placement. The primary spectral colors are produced 
as shown in Fig. 95. If a source of light S, a lens I, 
and a grating G, be arranged as shown, an image of 
the source will be seen on a screen at /. With a 




Fig. 95. — Illustrating the Wood diffraction process. 

fine grating a spectrum of the source will be seen on 
the screen extending between aa. With a coarse 
grating a spectrum of the source will be formed 
between cc and a medium grating will produce a 
spectrum at bb. If the three gratings have different 
rulings, the eye at E will see the lens face illuminated 
by a monochromatic color depending upon the grating 
interposed at G. If all three spectra be produced 
simultaneously in proper intensities, the eye at E 
would see the lens face illuminated by white light 
providing gratings of proper rulings are chosen. Such 
a scheme was used by Wood for viewing the photo- 
graphs. The latter, which appear colorless, really 
consist of images of the object made on plates of 
bichromated gelatine through three properly chosen 



COLOR PHOTOGRAPHY 217 

gratings. In making the photographs one of the 
gratings was placed in contact with the bichromated 
gelatine film and the image of the object was pro- 
jected upon the sensitive film through the grating. 
This grating was then replaced by another and the 
procedure repeated. It was then repeated a third 
time with the remaining grating, but usually with 
another sensitive plate. On superposing the two 
exposed plates and viewing by a proper combination 
of lens and light source the picture was seen in 
colors. Copies can be made by contact printing. 
It was found, however, that while satisfactory pictures 
could be made there was no certainty about obtain- 
ing them. This was later found to be due to super- 
posing the three grating exposures. Ives^^ improved 
the process by printing the grating pictures through 
a very coarse grating placed at right angles to the 
lines of the three gratings. The coarse grating had 
opaque lines twice the width of the transparent strips. 
After making an exposure through one of the gratings 
the coarse auxiliary grating was moved in a direction 
perpendicular to its lines a distance equal to the 
width of one of its open slits and an exposure was 
made through the second grating. This procedure 
was repeated for the third grating. The lines of the 
coarse auxiliary grating were as in the so-called Joly 
process to be discussed later, so narrow as to be just 
unresolved by the eye — about 200 to the inch. This 
process yielded satisfactory results. Ives further 
simplified the process by making one grating answer 
the purpose of the original three by using the finest 
grating — 3600 lines per inch — and rotating it in 
its plane respectively 21.5 and 42 degrees for the 
other two exposures. Thorp, unknown to Ives, had 
previously suggested the use of one grating for a 



218 COLOR AND ITS APPLICATIONS 

similar purpose. When using three gratings, one 
with 2400 lines per inch furnished the red component, 
one with 3000 the green, and one with 3600 the blue. 
F. E. Ives worked out a viewing apparatus involving 
important improvements over Wood's original scheme. 
60. Color Filter Processes. — If an object be 
separately photographed on three panchromatic plates 
respectively through properly chosen red, green, and 
blue filters (which collectively transmit all visible 
rays) and these three photographs be separately 
projected upon a white screen by means of three 
projection lanterns equipped with the foregoing colored 
filters, three separate * monochromatic' photographs will 
be seen. If, however, the three colored images 
— red, green, and blue — be superposed in exact 
coincidence, a picture in natural colors will be seen. 
The principle is that of adding colors as shown in 
Fig. 21. F. E. Ives, who was a pioneer in this field, 
developed an apparatus for viewing the three-colored 
photographs simultaneously and also the so-called 
chromoscope for tri-color projection of photographs 
made in this manner. Charles Cros independently 
developed a similar method. In 1868 Louis Ducos du 
Hauron described a process for three-color photog- 
raphy (since known as the Joly process) which in- 
volved the ruling of red, green, and blue lines of 
transparent dyes on a transparent screen. The lines 
were too fine to be distinguished by the eye. The 
procedure involves the juxtapositional method of 
color-mixture, a principle long used in the textile 
industry and in painting. If a photograph be made 
through such a screen and a positive made there- 
from, the latter will appear in colors when viewed 
through the original screen when properly superposed. 
The screen is diagrammatically shown largely mag- 



COLOR PHOTOGRAPHY 



219 



nified in Fig. 96. (In Figs. 96, 97, and 98 red, green, 
and blue are represented respectively by the hori- 
zontal lines and the diagonal lines running in direc- 
tions perpendicular to each other.) There have been 
numerous variations of this scheme commercialized. 
The Paget screen is illustrated in Fig. 97. An inter- 
esting development is the Lumiere process. Minute 
grains of dyed starch are used in a thin layer over 
a sensitive emulsion. Three batches of transparent 
starch grains are -dyed respectively orange-red, green. 



du Hauron 



Paget 



Lumiere 






FIG.% 



FIO. 97 



FIG. 96. 



^ 



Green Red Blue 

Illustrating three processes of color photography. 

and blue. These are mixed in such proportions as 
to give a mixture of neutral color and are spread on 
the plate in a single layer. A portion of the plate 
greatly magnified is shown diagrammatically in Fig. 
98. The light passes through the minute color filters 
of dyed starch before striking the plate. The plate 
is developed in the ordinary manner, and by chemical 
means the negative is converted into a positive. The 
reversal may take place in a bath of potassium per- 
manganate acidified with sulphuric acid and is later 
developed again in the same developer as used in 
the first development. After drying, the plate is 
varnished and the color photograph is ready for 



rrvT\r 



220 COLOR AND ITS APPLICATIONS 

viewing. The process is a very ingenious one and 
reproduces natural colors quite satisfactorily. A de- 
ficiency of the process of no great importance in most 

work is shown in Fig. 99. 
It is also of interest in 
showing the inability of 
040 OiO m 070 the eye to analyze colors. 

.^.WAVELENGTH The approximate trans- 

Fig. 99.-iUustrating the limitations of missiou of the three dves 

certain processes of color photography. "^ 

are diagrammatic ally 
shown. It is evident on photographing the solar 
spectrum that the dyes used are somewhat too 
monochromatic, because the colored spectrogram 
which consists of red, green, and blue bands 
shows gaps between the blue and green, and 
also between the green and red where little color 
is visible. For instance in a spectrogram of the 
mercury spectrum the yellow lines at 0.578/^ appear 
an orange-red. This defect is evident in a greater 
or less degree in the foregoing processes, depending 
upon the spectral character of the colored dyes. 
However, owing to the fact that colors ordinarily en- 
countered are far from monochromatic, this deficiency 
is unimportant and practically negligible in ordinary 
color photography. This defect is encountered with 
regret when one desires to reproduce spectra for 
demonstration purposes. For the latter purpose the 
methods employing the three colored transparencies 
about to be described are satisfactory. The fore- 
going methods are based upon the additive and juxta- 
positional processes of color-mixture. The processes 
using the minute color filters shown in Figs. 96, 97, 
and 98 have a disadvantage in loss of light. For 
instance, if the process be analyzed it will be seen 
that a red object will be recorded upon the photo- 



COLOR PHOTOGRAPHY 221 

graph in general in the proportion of one red patch 
to two black patches. That is, no red light will be 
transmitted by the minute blue and green filters, so 
in the final photograph these will appear as black 
spots. This is a decided disadvantage in the making 
of colored lantern slides for projection unless an 
exceedingly powerful arc lamp is available. A num- 
ber of processes employing subtractive method (#18) 
have been developed. Sanger Shepherd developed 
a method wherein three differently colored films such 
as are indicated in Fig. 20 are superposed in a single 
transparency. F. E. Ives was also a pioneer in this 
field. The process is identical in principle with the 
tri-color printing process in use at the present time, 
with the exception that in the latter case a black- 
white record is sometimes used with the three color 
records. Three negatives are made respectively 
through red, green, and blue filters from which posi- 
tives are made on special thin transparent films of 
celluloid coated with gelatine sensitized by immersion 
in a solution of bichromate of potash. The trans- 
parencies are each dyed a color complementary to 
that of the taking filter, the red record being colored 
blue-green (cyan-blue); the green record, purple 
(magenta); and the blue record, yellow. These 
transparencies are free from opaque silver deposit, 
the gradation being from a maximal transparency to 
the deepest color of the dye on each film. On super- 
posing them the natural colors are produced by the 
subtractive method, as will be readily understood 
from an inspection of Fig. 20. F. E. Ives devised a 
method after this principle whereby the three plates 
were exposed simultaneously with one lens. Shep- 
herd first employed a repeating plate holder so that 
the three plates were successively exposed through 



222 COLOR AND ITS APPLICATIONS 

the proper filters. A few years ago a process of 
producing moving pictures in colors known as Kinema- 
color was launched. In order to simplify the matter 
only two colors are used, namely a blue-green and an 
orange-red. The different colored images are alter- 
nately thrown on the screen at the usual rate. It 
is obvious that the use of three colors would render 
the problem exceedingly complex. Such a two- 
color method cannot reproduce all colors with fidelity, 
but the results are quite satisfactory considering the 
simplification that is obtained. Recently another 
scheme, employing only two colors, has been devel- 
oped, known as the Kodachrome process. By means 
of a repeating back two plates are successively ex- 
posed through red and green filters respectively. 
These are developed in the ordinary manner and 
after being washed they are bleached and fixed, at 
this stage appearing transparent. They are next 
given a final washing in a weak aqueous solution of 
ammonia and dried. Finally the plates are dyed, 
the one made through the red filter being dyed a 
bluish-green and the one made through the green 
filter an orange-red. 

In general the processes employing dyed trans- 
parencies superposed yield more brilliant color records, 
but are obviously more dependent upon the skill of 
the photographer. In much work the processes 
employing the juxtapositional method of color-mixture 
are more satisfactory owing to the simplicity, notwith- 
standing the less brilliant results. Of the latter 
methods those employing the ruled screens are some- 
what more flexible; however, the adjustment of the 
viewing screens requires some patience. 

It is thus seen that at the present time the prob- 
lem of color photography has been solved by rather 



COLOR PHOTOGRAPHY 223 

indirect methods involving color-mixture. Most of 
the methods will be completely understood on refer- 
ring to Chapter III. 

REFERENCES 

1. Wiedmaim*s Ann. 1895, p. 335. 

2. Amer. Jour. Sci. 1887, p. 349. 

3. Ann. d. Chimie et Phys. 1848, p. 451. 

4. Comp. Rend. 114, p. 961; 111, p. 575. 

5. Lehrbuch der Photochrome, 1868. 

6. Ann. d. Phys. 1899, 69, p. 488. 

7. Des Farbenphotographie nach Lippmann*s Verfahren, 1898. 

8. Die Photographie in naturlichen Farben, 1894. 

9. Beitrage zur Theorie und Praxis der director Farben-photo- 
graphie, 1906. 

10. Astrophys. Jour. 1905, 27, p. 325. 

11. Jour. Franklin Inst. June, 1906. 

OTHER REFERENCES 

Louis Ducos du Hauron, Les Colours en Photographie, 1868. 

R. Child Bailey, Photography in Colours, 1900. 

E. Konig, Natural Color Photography, 1906. 

E. Konig, Beiblatter Ann. d. Phys. 1909, p. 1027. 

J. A. Starcke, Sci. Amer. Sup. Mar. 9, 1913, p. 158. 

A. Byk, Phys. Zeit. Nov. 22, 1909, p. 921. 

G. E. Brown, Photo Miniature, No. 128, 1913. 

G. L. Johnson, Photography in Colours, 1914, 



CHAPTER XI 
COLOR IN LIGHTING 

61. Lighting is of great importance, because it is 
essential to our most important and educative sense 
— vision — and color is intimately associated with 
lighting and vision. Color in lighting is rapidly grow- 
ing in interest in the science and art of illumination. 
The recent increase in the luminous efficiency of 
light sources and the rapid strides in the development 
of the art of lighting are largely responsible for the 
growing interest in color and quality of light. Much 
is yet to be learned regarding the physiological and 
psychological effects of color, and the laws for its 
proper use are hazy and not well understood. How- 
ever, equipped with a full knowledge of the physics 
of color, an aesthetic taste and a comprehensive view 
of what is known and unknown regarding the physio- 
logical and psychological influence of color, a person 
is capable of utilizing many of the possibilities of 
color in lighting. The illuminant plays a very im- 
portant part in the appearance of colors, as has been 
seen in Chapter VII. The spectral character of the 
illuminant influences the hue and relative brightness 
of colors, and the intensity influences the hue and 
apparent saturation. At low intensities the hue 
shifts toward the shorter wave-lengths and at high 
intensities there is an apparent decrease of satura- 
tion. The distribution of the light affects the appear- 
ance of colors, owing to the character of these 
surfaces. All of these factors are of importance in 

224 



COLOR IN LIGHTING 225 

considering the proper illuminant for accurate color 
work in the dye-rooms of textile and paper mills, in 
the mixing of pigments for color printing and for 
painting, for the matching of colors, and in many 
other places. The spectral character of illuminants 
is of importance (#37) in the discrimination of fine 
detail, for it has been seen that monochromatic light 
is superior in defining power to light of any other 
spectral character. 

There are many important problems as yet un- 
solved which involve color in its application to light- 
ing. There are practically no data on the influence 
of color on eye fatigue, although it is known that 
colors are of influence psychologically. There is a 
prevalent idea that the kerosene lamp is *easy on the 
eyes,' owing to its yellowish color. However, the 
low intrinsic brightness of the kerosene flame as 
compared with more modern illuminants is a fact 
worthy of consideration. When it is further noted 
that there is no general objection to daylight on 
account of its color — and it is far whiter or more 
bluish than ordinary illuminants — it must be ad- 
mitted that the virtue of the kerosene lamp based 
upon its color is on a rather shaky foundation. It 
is likely that the eye having evolved under daylight 
is better adapted to it than to any other illuminant 
and that the nearer an artificial illuminant approaches 
daylight in spectral character the more likely is it to 
be satisfactory physiologically. Misuse of common 
illuminants is perhaps responsible for eye-fatigue to 
a greater extent than any spectral characteristics. 
One cannot look directly at the sun and state con- 
scientiously that daylight is ideal. It has been found 
that visual acuity is better in monochromatic light 
than in daylight (#37), and it may appear from this 



226 COLOR AND ITS APPLICATIONS 

that daylight is not ideaL However, these experi- 
ments were carried out at ordinary intensities con- 
sidered satisfactory in artificial lighting, and daylight 
intensities are ordinarily very much greater, which 
means that, for the discrimination of ordinary details, 
the intensity is many times the minimal amount re- 
quired, so that the limit of defining power is seldom 
reached. For years many have held that the eyes 
are less fatigued when reading from yellow paper 
than from white paper. In a biography of Joaquin 
Miller we read that *he wrote on yellow paper with 
a pencil because white paper hurt his eyes.' Bab- 
bage many years ago strongly advocated the use of 
yellowish paper in reference books, such as logarithm 
tables, where the eyes are severely taxed. Javel 
later advocated the same procedure, claiming that 
eye-strain was decreased, owing to a decrease in 
contrast. Many are of the same opinion although, 
as already stated, quantitative data relating directly 
to the problem are lacking. After reading from white 
paper the eyes seem to welcome a change to yellow 
paper, but this may be due to a decrease in contrast, 
owing to a lower reflection coefficient of the yellow 
paper than that of the white paper. However, meas- 
urements show only a slight difference in the bright- 
ness of pale yellow copy paper as compared with white, 
especially under ordinary artificial light. There is 
no doubt that a yellow or yellow-green light of less 
extended spectral character than daylight or ordinary 
artificial light is of superior defining power, due to 
the reduction of the effects of chromatic aberration 
in the eye. This fact may partly account for the 
contention that yellow paper is * easier on the eyes.' 
It is difficult to focus blue light at a normal reading 
distance, and impossible to do this at the same time 



COLOR IN LIGHTING 227 

keeping the most luminous rays in focus, therefore, 
the elimination of the blue rays by means of yellow 
paper may actually increase the definition. However, 
reading does not ordinarily involve the discrimination 
of fine detail, but instead the recognition of groups 
of characters. Furthermore, the eye is found to pro- 
gress across a page in a series of jumps, being sta- 
tionary only a few times per line. It has been found 
that there is practically no difference in visual acuity 
when the detail is viewed against a white ground 
and a ground consisting of yellow copy paper when 
both receive the same intensity of illumination, that 
is the same density of light flux. 

Colored surroundings, such as foliage, brick walls, 
the wall coverings of the room, etc., alter the spectral 
character of light before it arrives at the useful plane. 
Such effects must be considered in any lighting prob- 
lem requiring a light of a certain spectral quality and 
are also of importance from the aesthetic viewpoint. 
Many uses of illuminants of different color and colored 
media are found in the problems of lighting. 

^62. The Production of Artificial Daylight. —The 
arts having developed largely under daylight illumi- 
nation, the daylight appearance of colors is naturally 
considered as standard. With the production of 
artificial light man became less dependent upon day- 
light; nevertheless, owing to the impracticability and 
perhaps impossibility of a dual criterion of color, there 
has always been a demand for artificial daylight. 
The efforts in the production of artificial light have 
been directed toward the production of light of day- 
light spectral quality. The principal reason, no doubt, 
is that such a procedure in our most important method 
of producing light (by high temperature radiation) 
at the present time tends toward an ever-increasing 



228 COLOR AND ITS APPLICATIONS 

luminous efficiency. Nevertheless each increment in 
the steady approach toward daylight has been loudly 
acclaimed by reason of the better * color-value ' of the 
illuminant. However, there is a method which has 
been applied whereby light of a daylight character 
can be obtained by excluding from an illuminant con- 
taining all the rays found in daylight, those portions 
which are present in excessive amounts. Such a 
subtractive method is wasteful of light, but is made 
practicable by the recent increase in the luminous 
efficiency of illuminants. However, it is well to 
remember that efficiency in lighting as in any other 
case is *the ratio of satisfactoriness to cost and not 
the reciprocal of the cost' 

In order to produce artificial daylight it is neces- 
sary to determine the spectral character of natural 
daylight. First it is well to distinguish between sun- 
light and skylight. The latter is scattered sunlight, 
but owing to the relatively greater scattering of the 
rays of short wave-length (#13) skylight is more 
bluish in color than sunlight. Daylight varies tre- 
mendously with time and place, although north blue 
skylight and clear noon sunlight, when unaltered by 
reflection from immediate surroundings, are fairly 
constant in color. However, the modification due 
to selective absorption of the particles in the atmos- 
phere and selective reflection from foliage, buildings, 
etc., make daylight rather indefinite in spectral char- 
acter. E. L. Nichols ^ has published interesting 
accounts of his investigations on the spectral char- 
acter of daylight under different conditions of weather, 
cloudiness, location, and time of day. He found 
among other things unmistakable evidence of the 
coloring added to daylight by reflection from green 
foliage by noting the characteristic absorption spec- 



COLOR IN LIGHTING 229 

trum of chlorophyl (a substance in green foliage) 
present in observations made on land in the summer 
time. This effect was absent on the sea. Koettgen,^ 
Nichols and Franklin,^ Crova/ Vogel,^ Ives,^ and 
others have studied the spectral character of day- 
light. The data on noon sunlight and skylight plotted 
in Fig. 5 is a weighed mean of the results of the 
foregoing investigators as presented by Ives. The 
distribution of energy in the visible spectrum of 
clear noon sunlight as it reaches the earth corre- 
sponds closely to that of a black body at 5000 deg. 
absolute (C). 

A number of investigators, including Dufton and 
Gardner,^ Mees,^ Pirani, Ives,^ Hussey/" and Luckiesh^i 
have devised colored screens for producing artificial 
daylight by altering the light from an artificial source 
emitting a continuous spectrum. In order to demon- 
strate the procedure and illustrate the advantage of 
first choosing a light as close to daylight as possible, 
the productiion of daylight screens for two tungsten 
lamps of different luminous efficiencies as considered 
by Luckiesh and Cady ^^ will be presented. 

The visible spectrum of the light from a tungsten 
lamp being continuous, it has all the rays present that 
are found in daylight. The difference in their spec- 
tral characters is due to the difference in the relative 
amounts of the various rays present. First let us 
consider the production of light of noon sunlight 
quality from a vacuum tungsten incandescent lamp 
operating at 7.9 lumens per watt (1.25 w.p.m.h.c.). 
It is found sufficiently accurate to consider no rays of 
shorter wave-length than 0.42^. An ideal screen for 
altering the tungsten light to a noon sunlight quality 
will therefore transmit all the rays of wave-length 
0.42)U. It will partially absorb rays of longer wave- 



230 



COLOR AND ITS APPLICATIONS 



length in increasing proportions from 0.42^ toward 
the long-wave end of the spectrum. The reduction 
of the intensity of the rays of various wave-lengths 
is readily computed from the ratios of the amounts 
of these rays present in noon sunlight to the amounts 
of the corresponding rays present in the tungsten 
light under consideration. The resultant transmis- 
sion curve of a colored screen for thus altering the 



100 

90 

Q 60 

O 

tt: 70 

ixi 

■^ . 60 



^ 30 

20 



10 



V(7 ^^ ^""^^ 



040 "0.44 046 Q5Z 03& 0.00 0.64 0.66 0.7? 

Fig. 100. — Ideal transmission screens for producing artificial daylight. 



tungsten light (7.9 lumens per watt) to noon sunlight 
quality is shown in &, Fig. 100. The ideal transmis- 
sion curve of a colored screen for producing artificial 
noon sunlight by means of a nitrogen-filled tungsten 
lamp operating at 22 lumens per watt (0.5 w.p.m.h.c.) 
is shown in c. In order to produce artificial north 
skylight it is seen in Fig. 5 that the visible rays of 
long wave-length must be reduced by relatively 
greater amounts than in producing artificial noon 
sunlight. The ideal transmission curve for producing 
artificial north skylight by means of the tungsten 



COLOR IN LIGHTING 



231 



lamp operating at 7.9 lumens per watt is shown in a. 
The ideal transmission curve for producing artificial 
north skylight with the gas-filled tungsten lamp oper- 
ating at 22 lumens per watt coincides closely with b. 
That is, a screen which produces artificial noon sun- 
light with the older type of tungsten lamp operating 
at 7.9 lumens per watt will produce artificial skylight 
when used with the gas-filled tungsten lamp operat- 



100 

90 
u 80 



40 



20 



JO 






0.40 



044 048 



052 



056 



0.60 



0.64 



0.68 



0.72 



Fig. 101. — Showing the loss of light when using the ideal artificial-daylight 
screens with the tungsten lamp operating at 7.9 lumens per watt. 



ing at 22 lumens per watt. This fact has been taken 
advantage of by the author in developing daylight 
units. These curves show the increased daylight 
efficiency of the tungsten lamps operating at higher 
luminous efficiencies. This is further illustrated in 
Figs. 101 and 102. In the former Ei represents the 
luminosity curve of the eye for light from a tungsten 
lamp operating at 7.9 lumens per watt, that is, the 
relative light values of the rays of various wave- 
lengths. This curve may be found directly or by mul- 



232 



COLOR AND ITS APPLICATIONS 



tiplying the mean luminosity curve of the eye (Fig. 93) 
for equal amounts of energy of all wave-lengths by 
the amounts of energy of various wave-lengths in 
the spectrum of the light under consideration. In 
this case it is the 7.9 lumens per watt tungsten 
lamp whose spectral energy distribution is found in 
Fig. 5. On multiplying curve Ei by the transmission 
values of curve a, Fig. 100, curve a' is obtained. The 



100 
90 



>40 
§30 

UJ. 

or 

20 

10 

















































/ 




\ 


























/ 


f 




\ 


























/ 








\ 
























1 








\ 






















/ 










\ 


1 




















/ 












\ 




















/ 












\ 




















/ 




c' 










V 


















/ 


/. 




b' 




'N 


\, 




\ 














^ 


^ 


y 














5^ 


"^ 


^ 






040 



044 



04& 



Q52 



036 



0.60 



064 



0.6d 07? 



Fig. 102. — Showing the loss of light when using the ideal artificial-daylight 
screens with the tungsten lamp operating at 22 lumens per watt. 



areas under curve Ei and a' are proportional to total 
luminous sensations, and the ratio of the area of a' 
to that of El represents the skylight efficiency of the 
7.9 lumens per watt tungsten lamp as based upon 
the foregoing computations. The reduction in lumi- 
nous intensity when screen b is used with the source 
under consideration is found on comparing b^ with 
Eij in Fig. 101, and the ratio of the areas represents 
the sunlight efficiency of the 7.9 lumens per watt 
tungsten lamp. The corresponding data for screens 



COLOR IN LIGHTING 233 

b and c used with the 22 lumens per watt gas-filled 
tungsten lamp are shown in Fig. 102, where E. repre- 
sents the luminosity curve of the eye for this tung- 
sten light. Screen b produces skylight and reduces 
the luminous intensity an amount represented by the 
difference between the area of &' and E2 in Fig. 102. 
Screen c produces noon sunlight with an efficiency 
represented by the ratio of the area of c' to that of E2. 

The daylight efficiencies for the two lamps con- 
sidered in the foregoing were found by determining 
the relative areas. For the 7.9 lumens per watt 
tungsten lamp (vacuum type) the noon sunlight 
efficiency is 14% and the skylight efficiency 4%. 
However, for the 22 lumens per watt tungsten lamp 
(nitrogen-filled type) the corresponding values are 
considerably higher, being 25% and 13% respectively. 
It has been found in actual practise that the 
consideration of 0.42/i as the starting point for the 
computations just described conduces to a higher 
accuracy than necessary in most cases, therefore 
beginning with a screen of 100% transmission at 
OAd/jL the daylight efficiencies are very considerably 
increased. Under these circumstances for the 7.9 
lumens per watt lamp the sunlight and skylight 
efficiencies are respectively 18% and 9% and for the 
22 lumens per watt lamp 33% and 19%. 

It is thus seen that very accurate artificial noon 
sunlight can be obtained with an ideal colored trans- 
mission screen with the 22 lumens per watt lamp at 
an efficiency of 25% or at 5.5 lumens per watt. This 
is a higher efficiency than that of the ordinary carbon 
incandescent lamp operating normally at the present 
time. Artificial daylight sufficiently accurate for 
nearly all purposes can be made at a much higher 
eflaciency. The author has developed bulbs for the 



234 COLOR AND ITS APPLICATIONS 

high efficiency tungsten lamp that produce artificial 
daylight satisfactory for general illuminating pur- 
poses. Thus the advent of the high efficiency lamps 
has made artificial daylight available, and now that it 
is practicable it is surprising how many places are 
found for it. Besides in the general field of store 
lighting, artificial daylight is useful for mixing pig- 
ments, matching artificial teeth and buttons, cigar 
sorting, medical examination of manifestations of 
skin diseases, green houses where botany classes 
study at night, observations of chemical reactions, 
and for many other operations. 

The production of colored media for the above 
purpose requires spectrophotometric apparatus. Mis- 
takes have been made by using colorimeters or by 
using merely the eye to judge the color. As has 
already been seen, the eye is undependable for such 
purposes, because it is not an analytical instrument 
for the examination of color. Two lights may appear 
white to the eye, yet differ considerably in spectral 
character. For instance, ultramarine blue of a proper 
density will so alter tungsten light by transmission 
that a white paper will appear quite the same as 
under daylight, yet colored objects will appear greatly 
different. Such a screen is very useful for demon- 
stration purposes. The distribution of energy in 
the visible spectrum of a white light produced with 
an ultramarine ffiter screening a tungsten lamp 
operating at 10 lumens per watt as compared with that 
of noon sunlight, S, is shown in Z7, Fig. 103. This 
unit was once seriously proposed as a ^daylight lamp,' 
but was short-lived for the reason shown. Another 
white light is shown in curve C, which is produced 
by the addition of red and blue-green light. It is 
similar to the ultramarine white light, yet more ex- 



COLOR IN LIGHTING 



235 



treme. These three illuminants are called * white,' 
because a white object appears the same under all 
of them; however, a colored object does not. A 
quartz mercury arc will cause a white paper to appear 
nearly white, yet its spectral composition is known to 
consist chiefly of four lines in the visible region. 

130 
120 
MO 
100 
^ 90 



^ 80 



uJ 

UJ 50 

> 













..... 
















































/ 
































/ 










\ 


































\ 


















\ 














\ 


X 
















\ 
















\ 
















/ 


















\, 






5 








/ 












^ 


-- 


" — 


, — J 


\ 










1 








^ 


^ 










Q 


\ 


\ 








/ 




/ 


y 








/ 


^ 








N> 


^ 




y 




/ 












/ 










\ 








/ 


























■\j 


■^ . 


^ 













0.40 0.44 



0.48 



0.52 



0.60 



0.64 



0.68 



0.72 



0.56 

Fig. 103. — Showing the spectral analyses of two subjective white lights compared 
with the spectral analysis of noon sunUght. 



I 



These examples illustrate the importance of spectro- 
photometric measurements in such problems. 

Another method of producing daylight is to add to 
a contiguous-spectrum illuminant the correct amounts 
of certain rays which are not present in sufficient 
amounts. To most artificial illuminants of this char- 
acter violet, blue, and blue-green rays must be added. 
To illustrate the procedure the two tungsten lamps 
considered previously will be used. In Fig. 104 curve 
S represents the spectral distribution of energy in 



236 



COLOR AND ITS APPLICATIONS 



noon sunlight. Curves A and B represent respec- 
tively the spectral distributions of energy for the two 
tungsten lamps operating at 7.9 and 22 lumens per 
watt. These three curves are plotted with their 
energy values equal at 0.70/x, a point near the prac- 
tical limit of visibility for long-wave energy. By 
subtracting the ordinates of A and B respectively 

260 

240 

220 

200 

i75l80 

z: 

uJ 

t leo 



>I40 
^120 
UJIOO 

UJ 

^ 60 

40 



20 













































5^ 
















^ 










/ 


^ 




















> 












— 




^ 














y 














\' 








^ 






E 


Y 


/ 


/ 
















\ 


N, 




\ 


^y 


/ 


> 


/ 




















s 


\ 


,^ 


^ 


\/ 


/ 
























y 


N 


v/ 


/\ 


\ 




















y 


y 




/ 


>\ 


\, 


\ 


















y 


/ 


_^ 


y 






s 


\ 


\ 












,^ 


y 




^ 


y 












\ 


\ 












^ 




















\ 


\ 
































\ 





040 044 



048 



0.60 



0!&4 



0.68 0.72 



0.52 0.56 

Fig. 104. — Showing the additive method of producmg artificial daylight. 

from the ordinates of S and plotting the remainders, 
curves A' and B' are obtained. These curves are 
complementary to A and B respectively; that is, the 
light produced by A when added to the ligjit pro- 
duced by A' gives the same amount of light and of 
exactly the same spectral character as the light pro- 
duced by S, which is assumed to be white light. By 
multiplying the ordinates of S, ^4, and B by the light 
values of energy of corresponding wave-lengths the 
curves in Tig. 105 are obtained. For example, S is 



COLOR IN LIGHTING 



237 



the luminosity curve of the eye for noon sunlight. On 
integrating these curves the relative areas under S, 
5, and A are respectively 100, 50, 33. Thus it is seen 
that equal amounts of light from a nitrogen-filled 
tungsten lamp operating at 22 lumens per watt and 
light of such, a spectral character as B\ Fig. 104, will 
produce artificial noon sunlight. However one part 



240 


































220 














/ 


^T" 


\ 
















200 














/ 




\ 






■ 










180 












i 








\ 














ID 160 












/ 








\ 














^140 












/ 










\ 












o|20 












/ 






R 




V 












—I 
HlOO 












/ 




^ 




\ 


\ 












5 80 

LJ 

^ f.0 












/ 


/ 




A 


> 


\^ 




















/ 


/ 


/ 


^ 




N 


N^ 












40 










/ 


/ 


/ 








\ 








1 




20 








/ 


-U 


'/ 














^\ 








% 






^ 


^^ 


^ 
















^^ 


^ 






\Q 


04 


4 


04 


5 


0^ 


2 


0.5 


6 


o.e 





, 06 


4 


0.( 


lb 


072 



Fig. 106. — Showing the relative amounts of light of the character of A and B 
(Fig. 104) necessary to produce artificial daylight by addition. 

of light from a vacuum tungsten lamp operating at 
7.9 lumens per watt must be added to two parts of 
light of the character of A\ Fig. 104, to produce 
artificial noon sunlight. These data have proved of 
value in the use of colored lamps with clear lamps 
for the lighting of paintings and other decorative 
colored objects. 

In Table VII the *per cent white' values obtained 
by L. A. Jones ^^ for various artificial illuminants with 
a monochromatic colorimeter are presented. His 



238 COLOR AND ITS APPLICATIONS 

values show higher daylight efficiencies for the tung- 
sten incandescent lamps than obtained by Luckiesh 
and Cady.i^ The difference may be partly due to a 
difference in the standards of white light used and in 
part to the possible fact that the author's computa- 
tions were made for artificial daylight of too great 
accuracy. That is, it is possible that the extremely 
low luminosity of rays at 0.42)U makes it unnecessary 
to produce a screen that begins to absorb light at 
that extremely short visible wave-length. The com- 
putations for screens beginning to absorb rays of 
longer wave-length than O.dS/x more nearly agree with 
the data obtained by Jones. It is unfortunate that 
Jones did not rate his tungsten lamps in lumens per 
watt, which is more definite because the mean hori- 
zontal candle-power of a tungsten lamp depends 
so much upon the manner of mounting the filament. 
Ives 13 obtained data on the daylight efficiency of 
illuminants several years ago, but his standard of 
daylight used at that time does not agree with a 
standard later arrived at by him by weighing the 
observations of various investigators, so that his 
values are not presented here., 

63. Practical Units for Imitating Daylight. — Lu- 
minous efficiency in artificial daylight production is 
a minor matter in a unit developed for very accurate 
color-matching. However, there are many cases where 
light approximating daylight quality is desired for 
general lighting. Here the wattage is an important 
consideration, although illuminating engineers and 
consumers alike must learn that the efficiency of a 
lighting unit or installation is a measure of how well 
it fulfills its purpose. This means a broader concept 
than watts per square foot or effective lumens per 
watt. If a light source is used for illuminating dress 



COLOR IN LIGHTING 239 

goods, and blues cannot be distinguished from blacks, 
and greens as seen in daylight are confused with 
yellow and brown fabrics under the artificial light, 
then the efficiency of the lighting installation falls 
close to zero in these particular cases. As illuminat- 
ing procedure becomes more refined, and as the effi- 
ciency of light production increases, more attention 
is being given to the importance of quality of light, 
which is an important factor in many lighting prob- 
lems. For these reasons glassware for use with 
tungsten lamps of high efficiency was developed by 
the author 11 in 1914 which greatly improves the qual- 
ity of the light, and does so without such an excessive 
loss of light as would be impractical for purposes of 
general lighting. 

Three phases of daylight have been considered, 
with the result that three classes of units have been 
developed. The latest color-matching unit, in which 
the gas-filled tungsten lamp operating at 22 lumens 
per watt is used, produces light of a deep blue 
skylight quality at about 3 lumens per watt. With 
the multiple lamps of the same type operating at 15 
lumens per watt the light corresponds to that of sky- 
light not quite as blue, and the luminous efficiency is 
about 2 lumens per watt. This unit is used for the 
purpose of accurate discrimination of color in textile 
mills, laboratories, color-printing shops, etc. The 
colored screen is entirely of glass, and as there is no 
excessive temperature rise in a well-ventilated unit, 
the glass is permanent and the unit is entirely safe. 

The next class of units are intended to imitate 
clear noon sunlight. This might be considered an 
average outdoor daylight. There are many cases 
indoors where the daylight quality is a mixture of 
sunlight and skylight, and this unit is designed to 



240 COLOR AND ITS APPLICATIONS 

produce a satisfactory artificial sunlight at an effi- 
ciency of about 7 lumens per watt when multiple 
tungsten lamps operating at an efficiency of about 
16.5 lumens per watt are used. It will be noted 
that the luminous efficiency at which this artificial 
sunlight is produced is practically the same as that 
of the older type of tungsten lamps. Thus sunlight 
quality is available for general lighting purposes. 
The applications for such units are to be found in 
color factories, lithographing plants, wall paper and 
paint stores, paint shops, cigar factories, art galleries, 
etc. 

Other units have been made by combining this 
colored element with ornamental glassware, by casing 
with light-density opal, or by mixing the two inti- 
mately. These units are intended for use in general 
store lighting, where a better quality of light is often 
desirable than can be obtained from any practical 
light source available for general store lighting. Any 
desired step toward sunlight quality can be produced, 
the magnitude of the step, of course, depending upon 
the permissible overall luminous efficiency and the 
color desired. Notwithstanding the blue or white 
appearance of daylight, when such a quality of light 
is produced artificially, there is some objection to 
its use in stores because of the *cold' appearance, 
notwithstanding its necessity for the proper appear- 
ance of colors. By this means a quality of light 
better than can be obtained from any unaltered light 
source for general use is produced at a luminous 
efficiency sufficiently high to meet with favor. Ob- 
viously a quality of light approximately midway be- 
tween that from the new high efficiency tungsten 
lamps and sunlight can be obtained at a higher 
efficiency than that of the older types of tungsten 



COLOR IN LIGHTING 241 

lamps. Lamps operating at a higher efficiency emit 
a whiter light, to begin with, thus giving the gas- 
filled tungsten lamps a dual advantage over those of 
the older type for the purpose of artificial daylight 
production. Recently this glass, with a slight modi- 
fication, has been incorporated into bulbs for the 
gas-filled tungsten lamps for the purpose of general 
lighting. 

As already stated, any light source having a 
continuous spectrum, or one nearly so, can be used 
for the purpose of making artificial daylight. Other 
desirable characteristics are high luminous efficiency 
and steadiness of light both as to quality and in- 
tensity. The arc lamp early entered the field and 
has been used considerably, although fluctuations in 
both the color and intensity have been serious draw- 
backs. A unit developed by Dufton and Gardner ^ 
in 1900 appears to be the first practical use made of 
the colored screen for subtractively imitating day- 
light. Doubtless there have been many more or less 
approximate reproductions made by others. 

Many are familiar with the beautiful white light 
of the Moore carbon-dioxide vacuum-tube lamp.^^ 
No better approximation of average daylight could 
be desired; however, at present the luminous effi- 
ciency of the small units for color-matching purposes 
is quite low. Certain difficulties have prevented the 
general adoption of the longer tube, although wher- 
ever this unit has been used the quality of the light 
appears to be very satisfactory. 

In 1909 the mercury arc lamp was combined with 
the tungsten lamp in proper proportions, with the 
result that a white light was produced. However, 
this is only an approximate imitation of daylight, the 
blue lines • of the mercury spectrum supplying the 



242 COLOR AND ITS APPLICATIONS 

blue rays in which the old tungsten lamp was quite 
deficient. This combination cannot result in a true 
daylight as considered spectrally, because the spectrum 
of the mercury arc consists of only a few lines. The 
addition of the fluorescent reflector to the mercury 
vapor lamp greatly improved this illuminant by adding 
red rays, but this is done partially at the expense of 
green light. (See Figs. 4, 15, and 16.) 

Early in 1911 Ives and Luckiesh,^ by means of two 
commercial glasses and an aniline dye, produced a 
screen for use with the old tungsten lamp operating 
at 1.25 w.p.m.h.c. for the purpose of producing * aver- 
age daylight,' that is, noon sunlight. Later the two 
glasses were replaced by a single glass, but a cor- 
recting aniline dye was still necessary. 

In 1912 R. B. Hussey ^^ described a screen for 
use with an intensified arc which produced sunlight 
quality. This was done by means of pieces from two 
colored glasses arranged in a checkerboard fashion, 
with suitable diffusing glasses to mix the light. Owing 
to the unsteadiness of the arc, spectrophotometric 
measurements were difficult to make, therefore a 
colorimeter developed by F. E. Ives was used (#28, 
Fig. 53). It will be noted that colorimeter measure- 
ments are not sufficiently analytical for the purpose 
of determining the character of the spectrum of a 
light source. For instance, this instrument will indi- 
cate that the quartz mercury arc gives approximately 
white light, yet this light source emits a line spectrum 
consisting chiefly, in the visible region, of four spec- 
tral lines, as shown in Fig. 4. However, the colorim- 
eter measurements are of interest where the light 
is known to have an approximately continuous spec- 
trum. This instrument gives readings in terms of 
red, green, and blue components, which when mixed 



COLOR IN LIGHTING 



243 



produce the same color on a white surface as the 
illuminant under examination. In Table XVII are 

TABLE XVn 

Colorimeter Measurements on Units for improving the Spectral 

Quality of Artificial Light toward Daylight 





Colorimeter reading 


Source 
















Red 


Green 


Blue 


Average daylight (noonday sunlight) . 


100 


100 


100 


North blue skylight 


78 


82 


138 


Huss«y daylight arc lamp 


93 


111 


96 


Intensified arc lanip (hqrft) ," , . , 


147 


102 


51 


Ives and Luckiesh (artificial daylight) 


100 


93 


107 


Tungsten 1.25 w. p. m. h. c. (7.9 lumens per watt) .... 


183 


96 


21 


Tungsten 0.65 w. p. m. h. c. (16.4 lumens per watt) . . . 


164 


102 


34 


Tungsten 0.50 w. p. m. h. c. (22 lumens per watt) 


157 


103 


40 


Tungsten 1.25 w. p. m. h. c. in tinted reflector 


145 


103 


62 


Tungsten 0.65 w. p. m. h. c. in tinted reflector 


120 


102 


78 


New color matching unit (with 0.65 w. p. m. h. c. 








tungsten lamp) 


80 


84 


136 


Artificial sunlight units (with 0.7 w. p. m. h. c. 








tunFstfin latTip) 


110 


103 


87 







shown the results obtained with this instrument on 
Hussey's daylight arc and other data of interest 
comparable only in a rough manner. The daylight 
arc examined was a near approach to daylight as far 
as colorimeter measurements can be trusted, although 
it shows an excessive greenish component. This 
could be easily remedied. 

Sharp and Millar/^ in 1912, by means of colored 
screens arid tungsten lamps, also produced a daylight 
effect. About this time several units, designed to 
produce artificial daylight, appeared, but no examina- 
tion of these has been made and no quantitative data 
are to be found regarding them. 

The author ^^ has successfully used colored lamps 
combined with clear tungsten lamps by the additive 
method, as illustrated in Fig. 104. Blue, green, and 



244 COLOR AND ITS APPLICATIONS 

blue-green lamps were used with success for pro- 
ducing daylight effects in combination with clear 
tungsten lamps. A notable installation was the light- 
ing of the paintings at a large temporary art exhibit 
in 1913, where more than 400 colored lamps were used. 
This is perhaps the first large exhibition of paintings 
where any attempt has been made to produce a day- 
light appearance by means of artificial light. In 
order to produce a practical method for obtaining a 
light of better color value for lighting paintings and 
other colored objects, many experiments have been 
made,^^ with the result that, besides the glassware 
already described, metal reflectors have been used 
having a tinted surface of such a character as to alter 
the reflected light to a color complementary to the 
direct light from the tungsten lamp. Obviously this 
method results in altering the distribution curve of 
the reflector, producing in general a less concentrated 
distribution. This indicates that focusing and inten- 
sive reflectors of this character should be used 
instead of those of extensive type. The results 
obtained with tinted reflectors show that a very good 
quality of light is obtained at a loss of about 50 per 
cent of the original useful light. With coatings of 
less depth of color the loss of light is less, but the 
improvement in quality is also less. By changing 
the shape of the reflector the amount of the altered 
light can be varied within wide limits. For lighting 
mural paintings, for instance, the reflectors proved 
satisfactory. No attempt has been made to reproduce 
skylight or even sunlight, but a very desirable in- 
crease in blue and blue-green rays has been obtained, 
as shown in Table XVII. The same scheme has 
been applied to the prismatic glass reflector, a glass 
coating being applied in this case. 



COLOR IN LIGHTING 245 

In 1914 Ives and Brady ^ produced a glass for 
accurate color-matching for use with the Welsbach 
gas lamp or the tungsten lamp. 

Other units have been developed more or less 
approximating daylight, but some have not fulfilled 
the claims made for them. There appear to be two 
fields for artificial daylight units: one where accurate 
discrimination of colors requires a correct repro- 
duction of skylight, and another field where coarser 
color work is done, such as in the paint shops and 
lithographing plants. Light approximating sunlight 
quality has been found to fill the requirements in the 
latter field. 

The lighting of paintings is treated in Chapter 
XIII and other colored lighting effects in Chapter XII. 

65. Effects of Colored Surroundings. — The color 
value of illuminants has been a subject of consider- 
able discussion and investigation during recent years. 
Most of the work has been done with colorimeters, 
which, owing to their limited power of analysis, furnish 
data which are likewise limited. However, the light 
that reaches the object is ultimately of greater im- 
portance in lighting. This can be greatly altered by 
selective reflection from surrounding colored objects, 
but the effect has been a much neglected phase of 
lighting. G. S. Merrill ^^ measured the color value 
of daylight on the working plane in a room after some 
of the light had been reflected from the colored sur- 
roundings. The interior measurements were made 
on clear and cloudy days. They showed consid- 
erable alteration in the color of outdoor daylight. 
The author ^^ made a study of this factor in a minia- 
ture room lighted by means of a tungsten incan- 
descent lamp operating at 7.9 lumens per watt, green, 
yellow, and white wall papers in various combinations 



246 COLOR AND ITS APPLICATIONS 

on the walls and ceiling and direct and indirect 
lighting systems having been used. 

In order to illustrate the possible color change in 
light due to reflection from a colored surface, it is 
possible to take an actual case and utilize spectro- 
photometric data, but for simplicity we will take a 
hypothetical case. Assume a light source radiating 
equal amounts of monochromatic red, green, and blue 
light, and that this source is placed at the center of 
a hollow sphere the walls of which are covered with 
a perfectly diffusing green paper. The colorimetric 
analysis of the illuminant may be expressed as 

R G B 

100 100 100 

The reflection coefficients of this paper for the 
particular illuminant are assumed to be in per cent, 

R G B 

25.2 47.2 27.6 

The light received by the green paper in the sphere 
is reflected an infinite number of times. If the walls 
of the sphere are temporarily assumed to be white 
and if N is the reflection coefficient of the paper, 
then the total light falling on the walls will be 

Q=Q'+NQ+ N'Q' + iV <?' + . . . =Y^ (^) 

where Q = total light falling on the walls and Q^ = 
direct light from the light source falling on the walls. 
The color of a paper is generally determined by 
measuring the color of the light after it has been 
reflected once from the paper. It is seen that a total 
reflection coefficient of 33§% has been assumed for 
the green paper for this particular illuminant. The 
coefficient of reflection may vary within wide limits 



COLOR IN LIGHTING 247 

without any change in the color values. Based on 
the foregoing assumptions the reflection coefficient 
of this paper for the monochromatic red light is 25.2% 
of the original 100 units; 47.2% of the total 100 
units of green light; 27.6% of the total 100 units of 
blue light. For this case the total red, green, and 
blue components in the light incident on the wall 
paper after an infinite number of reflections will be 
respectively, 

<?R = Q'r + N^Q'^ + NIQ'^ + KQ'^ + ... = j-^ (2) 
<?G = Q'o + N^Q'o + KQ'g + mQ'o + ■■■= Y^^ (3) 

Qb = Q'b + N^Q'b + N^^Q'b + NiQ'^ + . . . = j-^ (4) 
and 

Q = Qr+ Qg+ Qb= total light on walls (5) 

Q' = Q'r + Q'q + Q'b = total direct light on walls (6) 

-^R) -^G) -^B ^^^ respectively the reflection coefficients 
for the monochromatic red, green, and blue compo- 
nents of the original illuminant. 

^rO'rj ^gQgj ^bQ'b are the color values of the 
wall paper as determined by a tri-color method of 
colorimetry under the light, Q\ 

Computations yield the results given in Table 
XVIII. On plotting these percentages (shown in the 
column on the right) in a color triangle, it is shown 
graphically as indicated in the table that the reflected 
light rapidly approaches pure green by successive re- 
flection, but of course the intensity rapidly diminishes, 
as is shown in Fig. 106. It is also instructive to plot 
the logarithm of the intensity against the number of 
the reflection which gives a straight line. All three 



248 



COLOR AND ITS APPLICATIONS 



TABLE XVm 

Computations According to Equations (2), (3), and (4), showing the Changes 

produced in the Light from a Special Source by Successive 

Reflections from a Green Paper 



The terms in 

Equations (2), 

(3) and (4) 


Actual values 


Percentages 


R 


G 


B 


R 


G 


B 


NQ' 

N2Q' 

WQ' 

WQ' 

N5Q' 

N6Q' 

WQ' 

N8Q' 

N9Q' 


100.00 
25.20 
6.35 
1.60 
0.40 
0.10 
0.03 


100.00 

47.20 

22.30 

10.45 

4.93 

2.33 

1.10 

0.52 

0.25 

0.12 


100.00 
27.60 
7.62 
2.10 
0.58 
0.16 
0.04 
0.01 


33.3 

25.2 

17.5 

11.3 

6.8 

3.8 

2.6 


33.3 
47.2 
61.5 
73.9 
83.4 
90.0 
94.0 


33.3 

27.6 

21.0 

14.8 

4.8 

6.2 

3.4 



compaiients decrease rapidly in intensity with the 
number of reflections, but the green component does 

100 
90 
80 
70 

^ eo 

g 50 

LU 

z 40 
30 
20 
10 



1 E 3 4 5 6 

NUMBER OF. REFLECTION 

Fig. 106. — Illustrating the effect of multiple selective reflections of light from 

a green fabric. 

not decrease as rapidly as the others. In Fig. 107 
are shown the relative values of the three components 
after various successive reflections. It will be noted 




COLOR IN LIGHTING 



249 



that the color of the light approaches saturated green 
as the number of reflections is increased. In the 
original paper various computations were made which 
relate to conditions of so-called indirect and direct 
lighting which will not be presented here. However, 
these indicate, as is shown by actual measurements 
described below, that the color of the walls and 
ceiling alter the color of the light in so-called indirect 
systems very much. 

100 

|O90 

z: 



£ 



0^^ 

R 



2 60 
o 

^50 
540 

d 10 



"0123450 

NUMBER OF REFLECTIOMS 

Fig. 107. — Showing the relative proportions of red, green and blue components 
in the reflected light from a green fabric after various successive 
reflections. 



Actual measurements were made in a miniature 
room illuminated by tungsten light (7.9 lumens per 
watt, 1.25 w.p.m.h.c.) of the color of the total light 
reaching the working plane. The room was four feet 
square and four feet high and the floor was assumed 
to be the working plane. The results are presented 
in Table XIX reduced so that the colorimeter read- 
ings for the tungsten lamp used in the investigation 
equal 100 for each of the three components. This 
course is considered legitimate inasmuch as only 



250 



COLOR AND ITS APPLICATIONS 



TABLE XIX 
Colorimeter Measurements in a Miniature Room imder 
Various Conditions of Surroundings 



Reduced Colorimeter, 
Readings 



Red Green Blue 



7. 

8. 

9. 
10. 
11. 
12. 
13. 
14. 
15. 
16. 



Tungsten lamp, 1.25 w. p. m. h. c. (7.9 lumens 

per watt) 

Tungsten lamp, 0.65 w. p. m. h. c. (17 lumens 

per watt) 

Carbon lamp, 3.1 w. p. m. h. c 

Carbon lamp, 4.0 w. p. m. h. c 

Color of dull yellow wall paper 

Color of dull green wall paper 

(Results with ttmgsten lamp, 7.9 Ixmiens per watt) 

Yellow walls and yellow ceiling, indirect 

Yellow walls and yellow ceiling, direct 

Yellow walls and white ceiling, indirect 

Yellow walls and white ceiling, direct 

Green walls and green ceiling, indirect 

Green walls and green ceiling, direct 

Green walls and yellow ceiling, indirect 

Green walls and yellow ceiling, direct 

Green walls and white ceiling, indirect 

Green walls and white ceiling, direct 



100 



100 



100 



78 


96 


126 


116 


104 


80 


129 


101 


70 


131 


115 


54 


104 


119 


77 


159 


111 


30 


143 


107 


50 


130 


107 


63 


111 


106 


33 


108 


139 


53 


109 


113 


78 


145 


128 


27 


119 


119 


62 


110 


102 


88 


106 


104 


90 



the relative magnitudes of the alterations on color 
are desired. For the sake of comparison the color- 
imeter readings in the same scale for other incan- 
descent lamps are presented. It is seen that ordinary 
wall paper of dull yellow color may alter the color 
of tungsten light so that the useful light is more 
yellow than the old carbon incandescent lamps. This 
is a factor too often neglected, and there are cases 
where lighting experts have striven to improve the 
color of artificial light by partially correcting glass- 
ware, yet this light was permitted to be largely re- 
flected from yellowish walls and ceiling. In stores 
and other interiors where attempts are made to cor- 
rect the artificial light the surroundings should be 



COLOR IN LIGHTING 251 

of a neutral shade or of a slightly bluish tint if this 
is compatible with the color scheme of decoration. 
Many possibilities arise where the tinting of light by 
reflection can be utilized, for, as is seen by the fore- 
going, the effect can be of considerable magnitude. 

65. Color in Interiors. — This subject is largely 
of interest to the decorator, and inasmuch as this 
book is chiefly confined to the science of color, the 
aesthetic side of color will not be considered except- 
ing in so far as lighting aids the decorator. Some first 
principles of interior decoration, however, may not be 
out of place here. A room has been likened to a 
painting: the floor representing the foreground; the 
walls, the mid-die distance; and the ceiling, the sky. 
A ceiling may be lowered apparently by treating the 
walls horizontally, that is by finishing the lower por- 
tions of the walls a dark shade and the next section 
a lighter shade to within two or three feet from the 
ceiling and permitting the ceiling finish to extend 
down the walls. Some decorators insist that color 
has much to do with the apparent size of a room, the 
lighter tints seemingly enlarging the room. 

The color of a room creates its atmosphere. No 
single color can produce the best effect any more 
than one note can produce a melody in music. It is 
the artistic variation in values and tints that satisfies 
the eye. The principles of masses, spaces, and con- 
trasts, as well as sequences in hue and brightness, play 
their part in harmonies of color. The law of appro- 
priateness is as important here as in other fields, yet 
color and brightness are largely matters of individual 
taste, thus limiting the artist in formulating rules 
which at best are not thoroughly understood. 

North rooms, or those shielded from direct sun- 
light, are in general more satisfactory when colored 



252 COLOR AND ITS APPLICATIONS 

in rose, cream, yellow, buff — the *warm' colors. 
Yellowish tints in the window curtains aid in giving 
the effect of sunshine. On the sunny side, rooms 
will perhaps be more satisfactory when colored pale 
blue, gray-green, or shades and tints of other *cool' 
colors. In introducing color into the illuminant by 
means of colored shades or lamps the color scheme of 
the room should be considered. Apparently many 
prefer bright red wall coverings, if one may draw 
conclusions from observations. This again is a matter 
of personal taste, but extremely pure and bright colors 
in lighting effects in interiors are to the author like 
living with a brass band. Many of the lighting effects 
in pure colors certainly arise from a lack of study of 
the use and influence of color. If a room is decorated 
for natural lighting, theoretically it should receive the 
same artificial lighting both as to direction and spec- 
tral character. Yet the change in the lighting — 
from natural to artificial — may be just the thing to 
relieve monotony. There are many statements on 
this subject that cannot be reconciled with the facts. 
For instance, a person may be satisfied with daylight, 
living under it from day to day without any other 
comment than that it is ideal. The same person, 
however, may object to the increasing * whiteness' 
of modern artificial illuminants. He insists that we 
must go back to the color of the carbon incandescent 
lamp, or even further to that of the candle flame. Is 
there a dual standard? Can daylight be satisfactory 
and the light of the tungsten lamp or Welsbach mantle 
be too * white'? As a matter of fact all modern illu- 
minants used in ordinary interiors — the gas and 
incandescent filament lamps — are in the same class 
and far yellower than daylight that enters interiors. 
Color is certainly the keynote of lighting in many 



COLOR IN LIGHTING 253 

interiors, but let us not base its use upon incorrect 
premises. If we prefer * warmer' colors in our arti- 
ficial illuminants, let us have them, but let us attribute 
this desire to the proper cause, which may be a love 
for change in color. Slight tints of rose and yellow 
may add something pleasing to the complexion, but 
deep yellow, orange, or red have an obliterating 
effect upon the flesh tints of the face. They also 
tend to make colors appear further from their daylight 
appearance than untinted artificial lights. Using color 
for color's sake is a legitimate procedure, and in the 
absence of sufiicient physiological and psychological 
data the use of color must remain, for the present, 
largely a matter of taste. In lighting it is well to 
bear in mind the effect of surroundings in coloring 
the useful light. 

Let us take a particular case — the use of amber 
glass with the tungsten lamp for aesthetic purposes. 
A combination fixture had an * indirect' bowl from 
which hung some direct units with yellow silk shades. 
The indirect light first passed through an amber glass, 
then after various reflections from ceilings and walls 
reached the useful plane. Inasmuch as the majority 
of living rooms have wall coverings tending toward 
the yellow, brown, buff, or so-called *warm' colors, 
the indirect component is likely to be considerably 
altered toward yellow in one of these rooms without 
the use of amber glass. If the wall coverings are 
of a * colder' tint why are they satisfactory under 
daylight and not under the far yellower artificial light? 
The result obtained with the amber glass would have 
been obtained without it by the use of a more yellow- 
ish wall and ceiling coverings. The color of the 
surroundings depends upon the spectral character of 
the illuminant. A yellowish paper may appear the 



254 



COLOR AND ITS APPLICATIONS 



same under a deep yellow light as a yellower paper 
under a pale yellow light. The object of these re- 
marks is to illustrate that there is some scientific or 
physical basis for discussing any alteration of the 
color of artificial light tending away from daylight 
in color. 

Inasmuch as amber glass is often used as in the 
foregoing, it is of interest to analyze it. The author 




^0.40 044 



046 



036 



o.eo 



0.64 



0.66 



052 
.AJL 
Fig. 108. — Screen for altering tungsten light to the same spectral character 
as carbon incandescent electric light, c, d, e show the transmission 
curves of amber glasses of different densities. 



has considered it unsatisfactory for the above pur- 
pose because of its greenish tinge, and has therefore 
sought for a yellowish glass or dye without this 
greenish tint. Inasmuch as amber glass is usually 
used for the purpose of altering the present illumi- 
nants to a color approximating the yellow light of the 
carbon filament lamp, kerosene or candle flame, let 
us take the case of altering the light of a tungsten 
lamp operating at 7.9 lumens per watt to the color 
of the old carbon lamp operating at 4 w.p.m.h.c. This 



COLOR IN LIGHTING 



255 



can be done at a loss of not more than 20% of the 
total light; that is, the tungsten lamp operating at 
1.25 w.p.m.hx. will, with a yellow screen, produce 
light closely approaching that of the above carbon 
lamp at about 1.5 w.p.m.h.c. Thus light similar in 
color to carbon incandescent lamp light can be ob- 
tained at a high efficiency with the tungsten lamp. 

Curve a, Fig. 108, represents the transmission 
curve of an ideal screen for altering the tungsten 



120 

110 

S 100 
to 

CO 

£ 90 
I .80 

uj 70 

> 

^ 60 
_j 
lu 
oc 50 













t.^^^'^^^^ 






















\ 


\f 










^ 












— 


— 


__. 


/-. 




'deo 


'I'L 


1 

/ 


^ 


- 


— 


_— 


. — 


— 


- 














,J 


/ 


























V 


r 


























\ 


i 




























^A 


r 




























7 








— 1 




















/ 

























40 

■ I I I I/I I I I I I I I 

040 044 048 0.5Z Q56 0.60 0.64 0.68 

M 
Fig. 109. — Comparison of ideal screen a, Fig. 108, with amber gh 



light to a spectral character close to that of the old 
inefficient carbon lamp. Curve c is the transmis- 
sion curve of a light-density sample of amber glass. 
Curves d and e are the transmission curves of thicker 
specimens of the same amber glass. Curve d is the 
result of reducing curve a at all points by 8%. It 
is seen that the amber glass is far from ideal, there 
being an excessive transmission of green light com- 
pared with the ideal curve a (or 6). As the density 
or thickness of the amber glass is increased the trans- 
mission of green rays decreases relatively more than 



256 COLOR AND ITS APPLICATIONS 

the yellow, orange, and red rays; that is, the dominant 
hue shifts toward red, which is apparent by casual 
observation. In Fig. 109 the transmission curve of 
the ideal yellow glass for the above purpose is plotted 
as a straight dashed line, and the transmissions of 
light and dark amber glasses relative to those of the 
ideal screen are plotted as shown. It is seen here, 
expressed analytically, what careful observation indi- 
cates to be true: that amber glass is far from ideal 
for altering modern illuminants to a color similar to 
that of the older illuminants which many claim to be 
the more aesthetic. 

By means of present day tungsten lamps used with 
a proper colored screen the light of the kerosene 
flame can be closely imitated at efficiencies from 5 to 
10 Itimens per watt depending upon the efficiency of 
the unaltered source employed. The light of the old 
carbon incandescent lamp can be imitated in the same 
manner at efficiencies from 7 to 13 lumens per watt. 
The author has treated this subject elsewhere.^^ 

Screens for this purpose are readily made by the 
use of dyes, although they will in general lack per- 
manency. Most yellow dyes are objectionable for 
the above purpose for the same reason as amber 
glass. Potassium bichromate is, under moderate 
conditions, a permanent yellow. To this may be 
added a pink dye, which will usually yield a com- 
bination which is satisfactorily yellow. For the pink 
some very dilute red dyes may be used. Rhodamine 
is satisfactory in color, but is very fugitive under the 
influence of light and heat. Many yellow dyes are 
quite permanent if used on a sheet of glass instead 
of directly on the lamp bulb, but usually these must 
be corrected as already indicated. 

Much pleasure can be derived from the use of 



COLOR IN LIGHTING 257 

tinted illuminants, for they lend themselves to deco- 
rative effects and afford an easy means for eliminat- 
ing monotony in lighting. Two- or three-circuit 
pendant units (preferably indirect or semi-indirect) 
are convenient for this purpose, for by using clear and 
colored lamps various combinations of color and in- 
tensity can be obtained which are very pleasing. 
Silk shades of various tints are readily applied to 
lamps, and colored gelatines are easily concealed and 
afford a ready means of obtaining pleasing colors. 
(Colored media are discussed in the last chapter.) 
Very brief descriptions of a few uses of colored light 
in interiors may aid in showing the possibilities of 
such application of the art and science of color. There 
are many indications that we are at the beginning 
of an age of color appreciation. It has already as- 
serted itself in modern painting; and the gorgeous 
display of color that greets the visitor to such mag- 
nificent architectural structures as the Congressional 
Library in Washington and the Allegheny County 
Soldiers' Memorial at Pittsburgh indicates that this 
century is likely to witness a renaissance in the 
use of colors in decoration. Color was the keynote 
in the plans of the Panama-Pacific Exposition i» 
much of which is obtained by lighting effects. Col- 
ored jewels reflecting millions of images of light 
sources, colored flames, moving color filters, and 
lights of various colors were woven into a gorgeous 
spectacle. W. A. D'a Ryan, who planned the color 
effects, has also used the * scintillator ' with con- 
siderable success. Powerful searchlights arranged 
to point radially upward illuminate clouds of steam 
in various colors. The beams diverge from each 
other in a fan-like manner. The possibilities in 
spectacular lighting are manifold. 



258 COLOR AND ITS APPLICATIONS 

A notable use of colored illuminants is found in 
the Allegheny County Soldiers' Memorial. In this 
splendid lighting installation, which was designed by 
Bassett Jones, ^^ mercury arc lamps, tungsten incan- 
descent lamps, Moore tubes, and yellow flaming arcs 
were used. The ceiling of the auditorium, which is 
sixty feet above the floor, is composed largely of glass 
in decorative panels. The central panel is outlined 
by means of the pinkish light of the nitrogen tube. 
Over the corner panels yellow flame arcs are hung, 
and their flicker adds charm to the colored ceiling 
which would not be present with perfectly steady 
light sources. The outer panels are lighted by the 
bluish light of mercury arc lamps, and tungsten lamps 
stud the ceiling, adding a touch of brilliancy. The 
contrasting of colors is so harmoniously accomplished 
that the result is exceedingly artistic. Thus the 
beauty of this monument of decorative art is visible 
at night as well as by daylight, which is too often not 
the case. There are many other interesting applica- 
tions of color which make this beautiful work of art a 
worthy mecca for those interested in color and lighting. 

Art galleries ojffer excellent opportunities for in- 
troducing the science of color lighting. As already 
mentioned, more than four hundred colored tungsten 
lamps were used with clear tungsten lamps in cor- 
recting the lighting of a temporary art exhibit. The 
results were extremely encouraging, inasmuch as they 
met with the approval of artists and critics alike. 
This was perhaps the first notable attempt ever made 
to furnish illumination of a daylight quality for light- 
ing paintings. This field offers a splendid oppor- 
tunity for development, which can readily be done 
by means of the color-correcting lamps and acces- 
sories now available. 



COLOR IN LIGHTING 259 

Many artistic effects can be obtained by the use of 
colored light in the home. A slight rose or orange tint 
in the light is very pleasing and attractive, although 
the choice of tints is of course a matter of taste. A 
rather interesting case is found in a dining room of 
a pretentious residence. A large oval panel of dif- 
fusing glass is set into the ceiling, and behind this a 
great many red, green, and blue lamps of low voltage 
are placed in the approximate proportions of two red, 
three green, and five blue lamps. The lamps of dif- 
ferent colors are controlled by means of dimmers set 
in the wall, so that by varying the proportions of red, 
green, and blue light various qualities of light may be 
obtained and also a large range of intensities. 

A person who enjoys color can readily devise 
many simple schemes for obtaining tinted light. An 
experiment which the author found of interest was 
the production of an artificial moonlight effect. A 
high decorative window in the living room was 
removed and placed in the normal position of the 
storm sash, thus providing space for tubular lamps 
in reflectors. The window was covered on the inside 
with a cardboard of bluish-green tint and in the open- 
ing before the window, a stained wooden lattice was 
placed, over which an artificial rambler rose was 
twined. The lamps, which were tinted a light blue- 
green, illuminated the bluish-green cardboard,- which 
as viewed through the foliage produced a charming 
effect of moonlight. As the space was narrow the 
cardboard was not uniformly bright, owing to the 
proximity of the lamp, but this defect was readily 
overcome by stippling the surface with a black water- 
color. Such effects are readily applied to bay win- 
dows and other convenient places. 

Many possibilities present themselves to those 



260 COLOR AND ITS APPLICATIONS 

interested in color lighting. The many colored media 
available and the diversity of the color of commercial 
illuminants provide the means for carrying out many 
ideas. Electrically excited gases, such as carbon 
dioxide, neon, helium and mercury vapor, contained 
in glass tubes, are commercial possibilities which have 
not yet been applied to the fullest advantage for 
elaborate colored effects. In the average case, how- 
ever, requirements are readily fulfilled by means of 
ordinary light sources and colored media. 

66. Color Preference. — It may be of interest here 
to record the results of some simple experiments, in- 
asmuch as such data may indicate eventually the 
effect of the illuminant upon our preference for certain 
colors and may throw some light upon the relation 
of lighting to the pleasing effect of colors. The ex- 
periments represent the beginning of an investiga- 
tion begun with an object in view which is discussed 
in Chapter XV but are described here as a matter of 
interest. The Zimmerman colored papers were used, 
but as there was no saturated green paper one was 
dyed and placed in the series. This is designated 
as g, the other letters indicating the catalogue desig- 
nation of the various colored papers. Fifteen col- 
ored papers, each four inches square, were spread 
out haphazardly upon a white surface, the individual 
papers being from six to ten inches apart. The ob- 
server was asked to study the colors and pick them 
out in the order of his preference. He was asked 
to isolate the individual colors from everything as far 
as possible, choosing the color for color's sake alone. 
In other words, if possible he was not to associate 
the colors with wearing apparel or anything else. The 
experiments were carried out under ordinary tungsten 
light (7.9 lumens per watt) and also under daylight 



COLOR IN LIGHTING 



261 



entering the window, in the latter case no direct sun- 
light being present. The intensity of illumination in 
each case was of such value as would be considered 
sufficiently high for viewing saturated colors. The 
two observations were carried out at least a week 
apart and usually several weeks intervened. The 
general consistency of the preference orders of the 



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DESIGMATIOM OF COLORED PAPERS 



o p m 
Fig. 110. — Showing the preference or rank of a number of fairly saturated colors. 



fifteen observers was somewhat surprising. The 
mean results are plotted in Fig. 110, the ordinates 
representing color preference. There may be some 
question regarding the legitimacy of the definition of 
color preference, but the procedure adopted here pro- 
vides a simple means of plotting the data. There 
being fifteen colored papers, the least preferred 
would be placed last and ranked fifteen, the highest 



262 COLOR AND ITS APPLICATIONS 

preference therefore being unity. It is seen that 
the least preferred colors were those of highest lumi- 
nosity and in general of lowest saturation. That is, 
purples and highly saturated colors having hues cor- 
responding to the regions near the ends of the visible 
spectrum, namely blue and red, were definitely fa- 
vored. This confirms a conclusion previously arrived 
at from other observations. 

According to E. B. Titchener^i there are two types 
of observers: one type prefers the saturated colors 
and the other definitely prefers unsaturated or * ar- 
tistic ' colors, but the former type constitute a majority. 
The author's observations indicate that, when colors 
are chosen for * color's sake' alone, the saturated 
colors are almost invariably chosen. E. J. G. Brad- 
ford,22 in experimenting with twenty-six university 
students with a set of fifteen papers each about 30 
inches square, found that saturated colors were most 
preferred. He also found that the admixture of a 
small proportion of another color lowered the posi- 
tion of the color in the preference order. Cohn^s 
has also contended that increase of saturation tended 
to make a color more pleasing. Bradford found that 
the order of preference remained reasonably con- 
stant by performing the same experiments on three 
observers after an interval of two weeks and again 
after a lapse of twelve months. The subject of 
color preference will be treated further in Chapter 
XV, but it may be of interest here to compare the 
results obtained by Bradford with those obtained by 
the author. In the latter's experiments nearly all 
the colors were as saturated as possible, while only the 
first eight of Bradford's were *pure.' Bradford does 
not state the character of the illuminant used, but 
presumably it was daylight, so the daylight preference 



COLOR IN LIGHTING 



263 



order taken from Fig. 110 is used for comparison in 
Table XX. 

TABLE XX 
Color Preference 



Rank 


Bradford 


Luckiesh 


1. 


Dark blue 


Dark blue 


2. 


Saturated green 


Blue 


3. 


Chocolate-brown 


Red-purple 


4. 


Pale blue 


Green 


5. 


Slate blue gray 


Violet-purple 


6. 


Saturated crimson 


Deep red 


'7. 


Pale green 


Orange-red 


8. 


Cofifee-brown 


Crimson 


9. 


Bluish green 


Dull yellow-green 


10. 


Ink-red 


Orange 


11. 


Cinnamon-brown 


Orange-yellow 


12. 


Pale pinkish brown 


Dull green 


13. 


Bluish green 


Slate blue gray 


14. 


Pink 


Yellow 


16. 


Yellowish Green 


Lemon-yellow 



A word of caution is necessary regarding drawing 
conclusions from Table XX. The colors are de- 
scribed so indefinitely and the two series of colors 
differed very much. In one series practically all 
colors were as saturated as it is possible to obtain 
them by means of pigments, but in the other series 
about half of the colors were tints and shades. For 
instance, in the latter series chocolate-brown is a 
saturated red of a dark shade. Furthermore, as 
seen by Fig. 110, the reds ranked fairly high, but in 
placing them in order, as in Table XX, they are near 
the middle of the list because several colors ranked 
just above them. Notwithstanding the foregoing 
there is a similarity in the two preference orders. 
Fig. 110 serves as an indication of the similarity of 
the preference order of the various observers. For 



264 COLOR AND ITS APPLICATIONS 

instance, there being 15 colors if every observer 
placed h (lemon-yellow) last, its rank would be 15. 
The mean rank for h was nearly 14, indicating that 
nearly all the observers placed it last. Dark blue 
was placed first by most of the observers. 

As far as the limited results indicate, there was no 
general difference in the preference orders under the 
tungsten light and daylight, excepting under the former 
illuminant the reds were definitely placed higher in 
the preference order than in daylight. This has 
seemed apparent from previous observations as well 
as the indication that of a series of saturated colors 
the most saturated are usually the most preferred. 
There is some indication from other experiments 
that the relatively few who prefer tints instead of 
saturated colors, when asked to choose the colors 
for color's sake alone, are those that are unable to 
overcome the tendency to associate the colors with 
other things. It is just this associational preference 
order that is of more interest in this chapter. That 
is, in lighting there is no doubt that tints are more 
proper or more aesthetic. The data which is dis- 
cussed in Chapter XV from the viewpoint for which 
they were obtained are inserted here merely to illustrate 
some points in the matter of color preference. The 
data on this subject are rare and the danger of draw- 
ing definite conclusions at the present time is clearly 
recognized. 

Observation during the past few years has led 
the author to conclude that in the matter of color 
preference for color's sake alone, the colors near the 
ends of the spectrum and the purple series are in 
general favored. Artificial illuminants are usually 
poverty-stricken in blue and violet rays. Therefore 
these colors can probably be made to appear more 



COLOR IN LIGHTING 265 

attractive by means of an illuminant having more blue 
and violet rays and less red and orange rays than 
ordinary artificial light. Strictly, the artificial day- 
light already described is in general the correct 
artificial illuminant, but experiments indicate that, in 
the illumination of colors for pure decoration, a * white' 
light in which violet and red rays predominate pro- 
duces very pleasing results. A glass of this char- 
acter was made of a proper density so that white 
objects had the same white appearance as under 
natural skylight, yet such color as the pinks, purples, 
blues, violets, deep reds, appeared richer. Inasmuch 
as in the decorative use of color, exactness in hue 
is not usually essential, and as the color is employed 
for our pleasure, it is legitimate to use the illuminant 
that produces the most pleasing result. It is well to 
have white objects appear white, yet if those colors 
which please us most can be made more pleasing by 
the use of * white' light of such a spectral character 
as described above, it is within the province of the 
lighting expert to use such a light. Cobalt-blue 
glass, in the absence of a specially made glass, will 
produce these results fairly well if chosen of proper 
density. An ultramarine blue screen used with ordi- 
nary artificial light will produce an extreme * white' 
light of this character. Prussian blue added to it 
forms a satisfactory screen for this purpose. In 
prescribing such an illuminant one is not committed 
to the opinion that the pigments used in ordinary 
decoration are not 'rich' enough to begin with. Such 
handicaps are not uncommon in many of the arts 
employing color, and furthermore colored decorations 
are often dimmed by exposure. In any event the 
matter is one that will be governed largely by taste 
and the adoption of such a lighting procedure as 



266 COLOR AND ITS APPLICATIONS 

indicated above is legitimate if it pleases those 
concerned. 

The foregoing experiments are not described here 
with the intention of suggesting that saturated colors 
should be used in lighting. They could not be used 
without endangering the appearance of many colors. 
These various comments have been made with the 
object of suggesting fields for thought and experi- 
menting. Of course it is realized that the matter of 
color preference is exceedingly complicated by all 
the phenomena of color vision and environment, yet 
the foregoing experiments are instructive if the limi- 
tations of the results are recognized. 

67. A Demonstration Booth. — The most effect- 
ive manner of studying and demonstrating lighting 
effects is found in the use of a booth specially de- 
signed for the purpose. Having employed such a 
booth for several years very successfully it appears 
of interest to describe one in detail. A number of 
different types have been constructed, but the one 
described here has been most successful. In Fig. 
Ill is shown the wiring diagram covering the prin- 
cipal features. The dotted line enclosing a rec- 
tangular space represents the front dimensions of 
the booth, the center being represented by the mal- 
tese cross. The lamps represented by the larger 
circles are placed in their relative positions. Fourteen 
clear 40-watt tungsten lamps indicated by numbers 
were spaced as shown around the inside of the box 
near the front side, thus providing light from various 
directions. These are controlled by a contact arm 
arranged to rotate. The control apparatus is dia- 
grammatically shown at the right, spread out for con- 
venience. These switches are actually placed in a 
small recess in the right end of the box, as shown in 



COLOR IN LIGHTING 



267 




I 



to 



Fig. 112. Twelve snap switches are shown above 
the rotating contactor, of which the upper six control 
clear lamps as indicated by the numbers. The middle 
switches Si and S2 in the upper two rows control, 
respectively, the four lamps on the left and right. 



268 



COLOR AND ITS APPLICATIONS 



These must be special switches and the wiring con- 
nections have been omitted for the sake of simplicity. 
The clear lamps are very useful in demonstrating 
effects of light and shade and for showing the effect 
of diluting colors, or decreasing their saturation, for 
which purpose a variable resistance is placed in series 
with the rotating contactor. Two single-pole double- 
throw switches are shown at the left of the rotating 



S-8- 



2-2- 



U (.) O 



o o o 



-I 

I 

r^ 

i~~ 

II 

II 

II 

II 

1 1 

ii 

lo 

J 



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I — ^ 



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I • 

o 



o 



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S \ N 

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Fig. 112. 



Showing dimensions and locations of lamps in the demonstration 
booth. 



contactor switch, which provide for quickly changing 
the lighting from above to below or from the left to 
the right side. A motor M, controlled by switch M\ 
is placed on an extension at the back of the booth, so 
that its elongated shaft can be projected through the 
back side in the center. (A motor operating on 
direct or alternating current and capable of rotation 
at a very high speed is desirable.) On this rotating 
shaff such experiments as those indicated in Figs. 14, 
23, 29, 30, 31 are readily performed. 

General lighting of many colors can be obtained 



COLOR IN LIGHTING 269 

on the objects placed at the center of the back 
from the rows of colored lamps — eight above and 
eight below — by controlling the relative intensities of 
the red, green, and blue lights by the corresponding 
rheostats indicated at the right. The switches con- 
trolling these lights are /?, G, and B in the bottom 
row of the twelve snap-switches at the right. 

The purity of the primary colors is of great im- 
portance. These can readily be made satisfactory 
by any one acquainted with coloring materials and 
the science of color-mixture. Perhaps the easiest 
procedure in most cases is to begin with a set of lamp 
colorings. Gelatine filters made as described in 
Chapter XVI may also be used; however, for demon- 
stration purposes colored lamps provide a more com- 
pact apparatus, although in the latter case constant 
care of the lamps is necessary, owing to the fading 
of the colorings due to heat and light. By using the 
rheostats the mixing of colored lights can be done on 
a white diffusing surface hung on the back of the 
booth at the center, which provides a very satis- 
factory means for the synthesis of colors. The effects 
of quality of light on colored objects can be readily 
demonstrated, and daylight effects can be easily 
shown by adding blue-green light to the clear tung- 
sten light. In fact practically any demonstration 
involving color-mixture is possible with such an 
apparatus. 

Single red (/?'), green (GO, and blue (50 lamps 
controlled by corresponding switches on the right, are 
placed as shown at the angles of an equilateral tri- 
angle, the green lamp being placed at the upper 
apex. Interesting colored shadow demonstrations are 
easily shown, the shadow experiment illustrated in 
Fig. 28 showing the primaries, complementaries and 



270 COLOR AND ITS APPLICATIONS 

white light having been developed for use with these 
lamps. Many of the effects described in this book, 
especially those in Chapter XII, have been developed 
in the booth just considered. Other electric circuits 
are also used, but these will readily occur to the 
experimenter. Two views of the booth are shown in 
Fig. 112 with dimensions. Those interested in such 
a field will find the use of such a booth exceedingly 
interesting and instructive. A number of booths, 
perhaps not as compact and universal, have been 
used by pioneers in the study of color effects. Sev- 
eral years ago Basset Jones, a pioneer in the art of 
lighting, employed such an apparatus in very inter- 
esting demonstrations. 

REFERENCES 

1. Jour. Franklin Inst. 1912, 173, p. 315. Phys. Rev. 26, p. 
498; 25, p. 123; Trans. I. E. S. 1908, p. 301. 

2. Wied. Ann. d. Phys. 1894, 53, p. 807. 

3. Silleman's Jour. 1889, 38, p. 100. 

4. Ann. d. Chimie et d. Phys. Ser. 6, 1889, 20, p. 480. Comp. 
Rend. 1889, 109, p. 493; 112, p. 1176, p. 1246. 

5. Berl. Berichte, 1877, p. 104; 1880, p. 801. 

6. Trans. I. E. S. 1910, p. 189. 

7. Brit. Assn. Report, 1900, p. 631. 

8. Lon. Ilium. Engr. 1912, 5, p. 79. 

9. Elec. World, 1911, 57, p. 1092; Lon. Ilium. Engr. 1911, 4, 
p. 394. Lighting Jour. (U. S.), 1913, 1, p. 131. 

10. Trans. I. E. S. 1912, 7, p. 73. 

11. Trans. I. E. S. 1914, p. 839; Elec. World, Sept. 19, 1914. 

12. Trans. I. E. S. 1914, p. 687. 

13. Bui. Bur. Stds. 1909, p. 265. 

14. Trans. I. E. S. 1910, 5, p. 209. 

15. Trans. I. E. S. 1912, 7, p. 57. 

16. Lighting Jour. (U. S.), April, 1914; Lon. Ilium. Engr. 
March, 1914. 

17. Proc. A. I. E. E. 1910, p. 1726. 

18. Trans. I. E. S. 1913, p. 61. 



COLOR IN LIGHTING 271 

19. N. E. L. A. BuL Feb. 1915, p. 87; Elec. World, 1915, 65, 
p. 391. 

20. Trans. I. E. S. 1913, 6, p. 9. 

21. Experimental Psychology, New York, 1910, p. 149. 

22. Jour, of Psych. 1913, 24, p. 545. 

23. PhiL Stud. 1900, 15, p. 279. 

24. Elec. Rev. and W. E. 1915, 67, p. 161. 

By M. Luckiesh: 

The Lighting Art, 1917. 
Artificial Light, 1920. 
Lighting the Home, 1920. 



CHAPTER XII 
COLOR EFFECTS FOR THE STAGE AND DISPLAYS 

68. The Stage. — It is not the intention to treat 
the use of colored light in stage and display effects 
as they have been practised heretofore, but to point 
out some interesting new possibilities that have been 
developed by the application of the science of color. 
By the use of red, green, and blue lights any desired 
color effect may be produced, but the purity of these 
primary colors is very important. Apparently there 
has been little exact science of color-mixture applied 
to the stage. It is true that wonderful effects have 
been produced, but it is just as certain that the pos- 
sibilities in color effects have scarcely been touched 
upon. The color effects of today have not passed 
beyond the play of colored lights upon colored scenes 
in a more or less haphazard manner, the final effects, 
which are often very attractive, being arrived at by 
a *cut and try' method. Examination of colored 
media used for such effects show that very often 
impure colors are used. In fact, satisfactory com- 
mercial colorings are rare, and it is usually necessary 
to alter them in order to obtain colored lights of 
satisfactory purity. As already stated, only pure 
primary colors — red, green, and blue — are essential 
for producing a large variety of colored effects. 

Colored effepts are based upon the principle that 
the appearance of colored objects depends largely 
upon the spectral character of the light which illumi- 
nates them; that is, the color of an object is not 

272 



EFFECTS FOR THE STAGE AND DISPLAYS 273 



inherent wholly in the object itself. Things are vis- 
ible only by virtue of the light which passes from 
them to the eye. For instance, a red fabric appears 
red because it has the property of reflecting only red 
light. Obviously, if red rays are not present in the 
light under which the fabric is viewed, it will appear 
black. Colors can be made to disappear on a light 
background if they are sufficiently pure and free 
from * black' by viewing them through a glass of 
proper color. Pale blue lines on white paper will 
practically disappear under a deep blue light, and red 
pencil marks on white paper will be invisible under 






(Red light) 



(Green light) 



(Blue light) 



Fig. 113. — Illustrating the effect of colored light upon the appearance of six 

colored papers. 

a pure red light of proper color. This principle has 
been applied in stereoscopic drawings, the picture 
for one eye being printed in blue-green ink and that 
for the other in red ink. On placing a blue-green 
glass before one eye and a red glass before the other, 
a stereoscopic effect is produced. 

In Fig. 113 are shown the relative brightnesses of 
six colored papers under red, green, and blue lights, 
the colored papers being in the same relative posi- 
tions in each group. The photographs were made 
through a very accurate filter specially made for the 
panchromatic plate used (Fig. 90), and therefore the 
brightnesses are shown as nearly in true relation to 
each other as the limitations of photographic repro- 



274 COLOR AND ITS APPLICATIONS 

duction permit. It is interesting to note some of 
the changes; for example, the two middle colors re- 
verse in brightness when respectively illuminated by 
red and green (or blue) lights. 

Carrying this principle further the author ^ has 
developed some colored effects which show promise 
of application as the applied science of color becomes 
more thoroughly understood and as the cost of pro- 
ducing suitable colored light decreases. In making 
these applications it must be remembered that a 
color is only completely defined when analyzed into 
the three factors, hue, saturation, and brightness. For 
the purpose of producing the disappearing effects to 
be described, a simpler analysis can be used. That 
is, it is convenient here to consider separately the hue 
of the light that the pigment reflects and the amount 
it reflects, the first involving the spectral hue and 
the latter its brightness (or value). A group of col- 
ored patches on a gray ground can be made to dis- 
appear — that is to become indistinguishable from 
each other and the background — when the colored 
patches are illuminated by light of such a spectral 
character that they reflect rays of exactly the same 
character and in equal amounts as the background. 
This condition will not hold for another illuminant; 
therefore, some of the colored patches will be dis- 
tinguishable under another illuminant. This dis- 
appearance can be produced in another manner. By 
using a light of such character that the colored patches 
will reflect practically none of it they will disappear 
if placed on a black or dark gray background. Both 
methods have been used in developing these colored 
effects. The success of the scheme depends largely 
upon the choice of pigments properly related to each 
other and to the colored lights employed. Pure 



EFFECTS FOR THE STAGE AND DISPLAYS 275 



transparent pigments are quite essential. In mixing 
the colors it is necessary to understand the principles 
of color-mixture, for in mixing pigments there is 
always a tendency toward black (Fig. 20). A large 
supply of pure pigments is desirable, so that a pure 
pigment may be selected instead of obtaining the 
necessary hue by mixture. For example green can 
be made by mixing yellow and blue-green. This 
subtractive method often results in a green plus black; 





Fig. 114. — Illustrating the changing of scenery by the use of colored lights. 

that is, a muddy green. If the green can be obtained 
directly as a pigment instead of by this mixture, the 
black component is not present. Attention to these 
finer points is what distinguishes the scientific colorist 
from those who arrive at results without heeding the 
fundamental principles. Owing to the confused state 
of color terminology and the indefinite notation of 
pigments, it is impossible to describe accurately how 
these disappearing effects are produced. They in- 
volve the science of color and can be produced readily 
if the principles are thoroughly understood. 



276 



COLOR AND ITS APPLICATIONS 



The modern tendencies toward the use of color 
and color effects point to great future possibilities 
in the application of the science of color. Already 
in some European theaters the stage scenery has 
been revolutionized, and lighting effects are playing 
a greater part in the drama than heretofore. The 
experiments described below suggest the possibility 



1 






Fig. 115. — Illustrating the disappearing effects produced on a specially painted 
scene by varying the color of the illiuninant. 



that rays of light, swift and noiseless, might take 
the place of some of the present-day cumbersome 
methods of scene-shifting. Possibilities are also sug- 
gested for representing the supernatural, heretofore 
unrealized on the stage. In Fig. 114 are shown, as 
well as can be represented in black and white, two 
appearances of a mountain scene. The mountain 
and entire background can be made to disappear at 
will by changing the color of the illuminant. The 



EFFECTS FOR THE STAGE AND DISPLAYS 277 

appearance on the left is that under the ordinary- 
yellowish light from tungsten incandescent lamps. 
The other appearance is that under an orange-red 
light. The colors in the foreground are violets, 
grays, blues, greens, and touches of yellow. Those 
in the background are white, yellow, orange, red, 
and pink. Lightning effects can be obtained by 
flashing reddish light on the painting. No attention 
was paid to congruity in the use of colors, for the 
painting was designed merely to illustrate the pos- 
sibilities of the scheme. Further striking effects can 
be obtained by the use of illuminants of other colors, 
especially pale blue-green light. Thus a scene can 
be changed by rays of light. It is also possible to 
make the mountain disappear and in its place to have 
some other scene appear, for instance a seascape. 

In Fig. 115 the first picture appears to be a Jap- 
anesque arrangement of foliage. This is the appear- 
ance under a deep orange-red light. Gradually, by 
introducing blue light, the figure appears, and on 
adding green light or clear light it appears fully in 
view. On extinguishing the red component in the 
illumination the figure, and especially the flowing 
robe, stands out in strong contrast and beautiful effects 
are produced by changing gradually from blue-green 
to a deep blue. By gradually introducing orange-red 
light and extinguishing the other components the 
figure slowly disappears. Such effects show the 
possibility in scenic effects in fairyland plays. It 
is well to understand that the photographic repro- 
ductions just shown only illustrate the brightness 
contrast. In the originals the contrasts are more 
striking, because they are due to differences in hue as 
well as in brightness. In fact, it is difficult to illus- 
trate in black and white the effects produced with this 



278 COLOR AND ITS APPLICATIONS 

particular subject, because in the center illustration 
most of the contrast is due to differences in hue alone. 

Another changing scene that was produced is that 
of a summer landscape gradually merging into a snowy 
wintry scene. By painting the body and branches 
of the trees a gray, and covering these and the ground 
with a bluish-green foliage, they appear in their 
abundant garb of summer under ordinary light. By 
changing the color of the illuminant to a *cold' pale 
blue-green the summer foliage disappears from the 
trees and from the ground, and barren trees and a 
ground covered with snow appears. These are the 
chief features of this scene. Of course touches of 
color added judiciously here and there greatly en- 
hance the beauty of the scene. Many other effects 
have been produced, but no attempt has as yet been 
made in applying them on a large scale in stage 
scenery. However, the problem in the theater is 
comparatively simple owing to the perfect control of 
the illumination. Certainly the possibilities of such 
applications of the science of color are very exten- 
sive. Only the simpler ones have been described 
here, owing to the necessity for demonstrating the 
principle as simply as possible. The more elaborate 
effects require more perfect interrelation of colors 
and illuminants. A field not to be overlooked is 
that of legerdemain, in which such disappearing and 
changing effects should prove valuable. 

69. Displays. — The foregoing effects are also 
applicable for advertising displays. In fact, it is 
strange that colored light has not been applied more 
to illuminated signs. Large tungsten lamps equipped 
with color filters and operated on flashers should add 
considerable to the attractiveness of ordinary scenic 
signs. The filters could be such as to produce moon- 



1 



EFFECTS FOR THE STAGE AND DISPLAYS 279 

light, daylight, and sunset effects upon a scene with 
great effectiveness. Colored lights pursuing each other 
in waves around the border of a sign represents 
a very simple application of colored light in adding 
movement to an ordinary illuminated sign. It seems 
that the introduction of changing and disappearing 
effects on illuminated signs should become popular^ 
owing to the lower cost as compared with the cost 
of an elaborately wired sign studded with incandescent 
lamps. There is no doubt that the latter signs are 
visible at a greater distance, but there are a large 
majority of signs that cannot be viewed from a 



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Fig. 116. — Illustrating a flashing sign produced by properly relating the hue 
and brightness of the pigments with the colors of the illuminant. 

great distance owing to obstructions. In Fig. 116 is 
represented a possibility outrivaling the ordinary 
illuminated sign, for actual disappearing effects are 
produced. The copy in the first view is in red, orange, 
and pale yellow. This disappears under orange-red 
light, the whole surface appearing of a uniform tint. 
The copy in the second view is in blue-green. On 
illuminating the sign with ordinary tungsten light 
the sign appears as in the third view. By alternat- 
ing with this clear tungsten light an orange-red 
light the copy shown in the first view appears and 
disappears. By alternating with the clear light a 
blue-green light the copy shown in the second view 



280 COLOR AND ITS APPLICATIONS 

disappears and appears. Thus various effects can 
be produced. This is a very simple sign, requiring a 
most simple wiring scheme. The flashing lettered 
sign has also been effectively combined with a scenic 
painting. Practically an endless variety of effects 
can be produced as rapidly as desired. Many other 
effects have actually been produced, such as a smiling 
and frowning face, a gesticulating speaker, a waving 
flag, and a rotating wheel, by this method of chang- 
ing the spectral character of the illuminant. These 
have been widely exhibited. 

This scheme has already been applied in displays. 
The haphazard play of colored lights upon colored 
patterns not designedly chosen is productive of catchy 
attractiveness; however, the actual disappearing ef- 
fects are more striking. For window displays the 
copy is placed in a darkened recess resembling the 
demonstration booth described in #67 so that it is 
protected from extraneous light. The colored lights 
are operated by flashers designed to bring about the 
proper sequence of appearances. Such demonstra- 
tions have been built and successfully operated. 
Successful experiments have been carried out with the 
color effects appearing by transmission through trans- 
lucent glass, in this case the colored lights being 
behind the glass. The colored scene or pattern is 
painted in transparent colors related as before on the 
back of the glass. It is also possible to project the 
colored light from a distance by means of parabolic 
reflectors, which would be an advantage in some cases 
for out-door displays. 

From the foregoing simple illustrations several 
advantages in the scheme are obvious. Apparent 
motion is obtained without elaborate wiring or me- 
chanical devices excepting the usual flasher. Copy 



EFFECTS FOR THE STAGE AND DISPLAYS 281 

can be changed continually if desired or a sign can 
be repainted often. The greatest difficulty lies in 
the initial preparation of the colors in proper rela- 
tion as to brightness and hue. In order to produce 
elaborate effects it will perhaps be necessary to use 
water colors or carefully prepared oil paints, protect- 
ing these with a covering of weatherproof varnish. 
The only expense involved in the change of copy is 
in the painting of it, for the color scheme can always 
be retained in proper relation to the colored illumi- 
nants. A flashing sign of this character is very 
simple, and the possibilities of scenic effects are 
greater than in any other method, simplicity being 
taken into consideration. The scheme adds to the 
possibilities of stage effects where it can be carried 
out with ease and sometimes be employed to sup- 
plant the jarring interruption due to shifting scenery. 
Of course the necessity of screening extraneous light 
if present will be a disadvantage in the application 
of this scheme to out-door displays, but there are 
many places where this will be unnecessary, because 
numerous bill-board sites can be found where there 
is little or no scattered light. 

The possibilities of the use of colored light in 
applying the science of color to displays, advertising 
and stage effects, have barely been touched. With 
the increasing efficiency of light production the utiliza- 
tion of color in lighting effects will become more 
elaborate. 

REFERENCES 

1. Elec. World, April, 1914. 
Amer. Gas. Inst. 1913. 
Lon. Ilium. Engr. 1914, p. 158. 
International Studio, April, 1914. 
Gen. Elec. Rev., March, 1914, p. 325. 



CHAPTER XIII 
COLOR PHENOMENA IN PALNTING 

70. Visual Phenomena, — The artist has often 
shown an antipathy toward science, apparently under 
the impression that art goes further than the mere 
mixture and grouping of colors and shadows and 
produces effects beyond scientific explanation. By no 
means is it contended here that art can be produced 
by *rule of thumb,' or by scientific formulae. Never- 
theless, facts are the basis of all art and, while 
scientific investigation has not yet revealed all its 
hidden secrets, scientific explanations can be pre- 
sented for many supposedly mysterious effects. It 
is proposed in this chapter to present the results of 
analyses and to indicate that science has been a great 
aid to art, and that it will perhaps render a much 
greater service in the future. 

The artist is in reality a link between two light- 
ings. He endeavors with chisel or brush to record 
an expression of light. The record is therefore an 
expression of light. Inasmuch as both the original 
scene and the painted record make their appeal 
through the visual sense, it is well to inquire into the 
process of vision. Seeing involves the discrimination 
of differences in light, shade, and color. In the ordi- 
nary sense no eye ever sees more, and no painting 
however * soulful' has more for its foundation, than 
differences in light, shade, and color. (In the general 
sense white, gray, or black are colors of complete 
unsaturation and varying brightness. It will perhaps 

282 





of the iliuiiaiuiiii 
. above 



CHAPTER Xin 

COLOR PHENOMENA IN PAIN 

Visual Phenomena. — The artist l. 
i an antipathy toward science, apparen 
mpression that art goes further than L 
mixture and grouping of colors and shadows and 
produces effects beyond scientific explanation. By no 
ttieans is it contended here that art can be produced 
by *rule of thumb,' or by «:r •-nHfir fArmnlae. Never- 
theles?;^ f^cts are the ' nnd, while 

scient 



aid 


and to indicate 


jeen a great 
pa render a much 


P 







o^tween two light- 
^A or brush to reco^*^ 
The record is ther 
smuch as bo U 



_^aire into th 



mgr. 

ai. ^rfr 

expi'^bJSiOii oi 

scene and th^; , - ' * 

through the visual :.. 

process of vision. Seeirig v: the discriniinatio:> 

of differences in light, shade, and color. In the ordi 

nary sense no eye ever sees more, and no painting 

however 'soulful' ^-* - -^^ ^"" 'ts foundation, than 

differences in lighi, r. (In the general 

sense white, gray, or black are colors of com? ie**? 

unsaturation and varying brightness. It will pe 

982 





Plate IV. Illustrating the effect of spectral quality of the illuminant. 
Daylight, below; ordinary artificial light, above 



COLOR PHENOMENA IN PAINTING 283 

be more convenient in this chapter to use the terms 
* colors' and * values' but with a clear understanding 
that the term * color' is here used in a restricted sense 
and that value is in reality included in the term *color' 
as used heretofore. See Chapter IV.) The funda- 
mentals of a painting therefore are colors and values. 
It is by grouping these elements that the artist makes 
his appeal to emotional man. However, science can 
aid the artist by analyzing the influences which alter 
these fundamentals. 

It took the artist many years to learn that the eye 
is far less perfect in definition than a simple lens 
and screen. In other words everything in the whole 
visual field is not seen distinctly at the same time. 
Definition is best at the point of the retina where the 
optical axis of the eye meets it, but outside of a small 
area surrounding this point, objects are not seen dis- 
tinctly. Further, the eye sees only the beginning and 
end of an ax stroke and it does not see all the move- 
ments of a galloping horse or splashing water. Pho- 
tography was hailed by many as being a useful means 
for recording a scene. But photography has done 
much to teach the artist what he should not paint — 
and that is the realistic picture recorded by the pho- 
tographic plate. Thus it is seen that material facts 
are often represented by artistic lies; that is, in 
reproducing a scene the artist does not record what 
he knows to be there, but rather what he sees. In- 
stead of recording details over the whole scene, the 
artist's task is to paint what the eye sees and in 
addition, by a sort of legerdemain, to record in colors 
and values, as far as is within his power, the impres- 
sions gained through the other senses. Thus the 
problem grows more complex, departing from the 
physical and entering the physiological and psycho- 



284 COLOR AND ITS APPLICATIONS 

logical realms. The physical laws are comparatively 
well understood, but the phenomena underlying the 
other fields are still hazy, owing to the lack of sufli- 
cient experimental data. In viewing a painting the 
problem becomes still more complex, for what the 
observer sees in a painting he must largely supply 
himself through the associational mental process. 

There are many vague terms used by artists, per- 
haps definite to those who use them, but the lack of 
systematic usage is confusing. It has been seen 
that the eye is far from being a perfect optical instru- 
ment. One of its faults is chromatic aberration; 
that is, an inability to focus different colors at the 
same time (#33). Naturally when viewing a group 
of different colors the eye is focused for the brighter 
colors. The eye is also constantly shifting under 
normal conditions. We are not conscious of these 
minute involuntary movements, but this shifting surely 
influences the appearance of paintings. The effects 
of after-images are also of importance (#43). If one 
views a red line on a green or blue ground, the effect 
is that of unrest. Both colors may not be exactly in 
focus at the same time, but perhaps of greater im- 
portance are the effects of overlapping after-images 
caused by involuntary eye-movements, which result 
in a *lost edge.' The latter effect is sometimes very 
striking at the horizon of a landscape painting. The 
after-image caused by a green stimulus is an un- 
saturated purple or pink. At the edge where green 
foliage meets a gray or pale blue sky a hazy pink 
fringe is often seen. The eye, in shifting slightly 
up and down, causes an overlapping of these after- 
images (approximately complementary to the original 
stimulus), thus forming a * lively' edge. The result 
of an after-image sometimes is to alter the saturation 



COLOR PHENOMENA IN PAINTING 285 

as well as the hue of a colored area. The phenom- 
enon of simultaneous contrast (#44) is very influen- 
tial, and of course is carefully studied by the artist. 
This effect of one area upon another is of consider- 
able magnitude under some conditions. Two adja- 
cent colored areas can mutually so influence each 
other that they each appear differently in hue, sat- 
uration, and brightness than if viewed separately. 
On considering these influences and those due to 
intensity and spectral character of the illuminant, it 
becomes evident that no color has any definite and 
fixed appearance after it is out of the tube. These 
are facts which should be of great interest to the 
artist. Indeed, the great artists understood some of 
these influences very well. Most artists recognize 
many of them, but in general some of the influ- 
ences are unknown to the vast majority of painters. 
These various phenomena are treated elsewhere. (See 
Plate III.) 

71. Lighting. — Light has been termed the soul of 
art. The body of a landscape consists of the material 
things, but as Birge Harrison states, 4ts soul is the 
spirit of light — of sunlight, of moonlight, of star- 
light — which plays ceaselessly across the face of the 
landscape veiling it at night in mystery and shadow, 
painting it at dawn with the colors of the pearl-shell, 
and bathing it at midday in a luminous glory.' But 
of scarcely less importance is the lighting of the 
painted record of an expression of light. In various 
chapters it has been shown what a great influence 
the illuminant exerts on colors and values — the very 
essence of a painting; however, slight attention has 
been given to this important factor. ^ The lighting 
artist should be to art what the musician is to music. 
His duty is to render the color symphony as the 



286 



COLOR AND ITS APPLICATIONS 



composer intended it to be rendered. This only can 
be done exactly by lighting the work both as to dis- 
tribution and spectral character of light just as it was 
lighted when the artist gave to it the final touch. 
This of course is impossible, but it is easy to light it 
by an artificial daylight which will render its appear- 
ance more nearly that which is had when completed 
by the artist, and to overcome certain limitations of 
pigments by properly distributing the light. (The 



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COLORED PAPERS 
Fig. 117. — Showing the reflection coefficients of fairly saturated colors for 
daylight and tungsten incandescent electric light. (See Table XV.) 



influence of the illuminant is seen in #42, 68, and 
Table XV.) Not only does the spectral character 
of the illuminant alter the hue, but likewise the bright- 
ness or value. In Table XV it is seen that pigments 



COLOR PHENOMENA IN PAINTING 287 

differ tremendously in reflecting power or relative 
brightness when illuminated with daylight and ordi- 
nary artificial light. These data have been plotted in 
Fig. 117 to emphasize this important point. The 
reflection coefficients of the various colored papers 
for daylight are shown by the dashed line /?d and 
for the light from a (vacuum) tungsten lamp operating 
at 1.2 w.p.m.h.c. by the full line Rj. The ratios of 
the reflection coefficient under tungsten light to that 
under daylight are shown by the upper curve. It 
will be noted that those pigments which predomi- 
nantly reflect red, orange, or yellow rays are con- 
siderably brighter under the tungsten light than under 
daylight, but those pigments which predominantly 
reflect violet, blue, and green rays are brighter under 
daylight. The curve shows that these ratios vary 
from 0.69 to 1.57; that is, some of these pigments 
change in relative * value' more than 50 per cent. 
However in painting, the relative * values' of adjacent 
and other areas are of importance, and such changes 
in relative brightness are often as high as 100 per 
cent. For instance, assume that clouds are adjacent 
to an area of blue sky in a certain painting and that 
these pigments are represented respectively by e and 
n. The ratio of the brightness of the clouds to that 
of the sky is 1.6 when the painting is illuminated by 
daylight. Under the light from the tungsten lamp 
this ratio is 2.8 or nearly doubled. In fact cases 
have been found where such ratios have doubled, 
as will be seen later. The hue changes are in some 
cases enormous, but these cannot be readily shown 
here. The artist recognizes the difficulty of painting 
under artificial light, yet he apparently does not raise 
his voice in protest when his work is illuminated by 
artificial light. In Fig. 118 are shown the effects of 



288 



COLOR AND ITS APPLICATIONS 




-a 



o 




-a 



•3 




bO 






COLOR PHENOMENA IN PAINTING 289 

different illuminants upon the values of a frieze 
painted with ordinary water colors. The illuminants 
were daylight, ordinary tungsten light (vacuum in- 
candescent lamp), and an orange-red light. In this 
frieze the upper rectangles were alternately tinted 
a reddish purple and a bluish purple. The lotus 
flowers and buds were tinted a pale blue, the stems, 
dark green, and the alternate sectors of the lower 
circular patterns were colored respectively a yellow- 
ish orange and a reddish orange. The background 
was white. An extreme example is shown in Fig. 
113. An example of the difference in the appearance 
of a painting under natural and ordinary artificial 
light is shown in Fig. 119 (see Plate IV). A photom- 
eter was used to measure the relative brightnesses 
of adjacent patches of pale blue sky and pale yellow 
clouds at about the center of the sky area in this 
picture, indicated by the circle. Under tungsten light 
the two patches were of equal brightness, but under 
daylight — the light under which practically all paint- 
ings are done — the patch of sky was twice as bright 
as the adjacent clouds. The filter used in taking 
these photographs was quite accurate, so that the 
values are faithfully represented. It is seen that 
the sky was much brighter than the foreground when 
the painting was illuminated by daylight. It is only 
fair to state that the difference in foregrounds is due 
somewhat to the lack of sufficient range of grada- 
tion in the photographic paper. Note also the relative 
brightnesses of the low-hanging clouds. If a paint- 
ing will stand such enormous changes in the relative 
* values' of its parts (aiid the accompanying changes 
in hue), it is indeed flexible. Under most artificial 
illuminants the hues in a painting shift toward the 
red as compared to their appearance under daylight 



290 



COLOR AND ITS APPLICATIONS 






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COLOR PHENOMENA IN PAINTING 291 

illumination. That is, a deep yellow appears orange, a 
bluish purple appears a reddish purple, blues and 
violets approach gray, and the reds are relatively 
brighter. Accompanying this shift in hue is a cor- 
responding shift in brightnesses or values. That is, 
yellow, orange, and red appear brighter, and violet, 
blue, and green appear relatively less bright, as shown 
in Fig. 117. 

The distribution of light on a painting has a great 
influence upon the expression of the painting. Meas- 
urements show that the range of relative brightnesses 
in a landscape is often as high as five hundred to one. 
That is, the brightest spot (for instance, cumulus 
clouds receiving direct sunlight) are often several 
hundred times brighter than the deepest shadow. 
The pigments employed by the artist will hot record 
such a physical contrast. In any landscape painting 
the brightest spot is seldom more than forty times 
brighter than the darkest spot when both receive 
practically the same amount of light as is usually 
approximately the case. A white paper is no more 
than fifty times brighter than a so-called black paper. 
In order to overcome this handicap due to the limi- 
tations of pigments the artist may resort to illusions 
if possible. For instance, a highly illuminated red 
object is not painted red but an orange-red, because 
it is true that under intense illumination colors appear 
less saturated. Thus, by painting the highly illumi- 
nated red object an orange-red, the illusion of in- 
tense illumination is produced. A hot desert scene 
is depicted in the same manner, with the additional 
illusion of short or minimal-length shadows. Thus 
the feeling that the sun is at the zenith helps to 
produce the illusion of a hot desert scene. 

R. W. Wood performed an interesting experiment 



292 COLOR AND ITS APPLICATIONS 

in accentuating contrast in a painting by projecting a 
positive lantern slide image of a painting upon the 
original in exact coincidence. In this manner the 
high lights received relatively very much more light, 
and the shadows less light than in the ordinary case, 
where the painting is uniformly flooded with light. 
This scheme, though interesting and instructive, can- 
not be used in practise. Extensive experiments on 
the effects of distribution of light over paintings indi- 
cate that a proper distribution is a legitimate and 
an effective aid to the artist in bringing forth the 
proper expression of a painting. In Fig. 120 are 
shown some effects of different distributions of light 
on a painting, although the limitations of the photo- 
graphic process prevent a very satisfactory illustra- 
tion of these effects. It is seen, however, that the 
mood can be changed enormously by altering the 
distribution of the light. The scheme is difficult to 
carry out in cases where the wall space is crowded 
with paintings, and it is unfortunate for various rea- 
sons that such crowded conditions must exist. How- 
ever, the principle is easily applied to individual 
paintings, and at the same time a correction of the 
light to daylight quality can be made. This has also 
been carried out in the case of trough lighting, which 
is often a practical and convenient procedure, because 
most paintings have their chief high lights in the upper 
portion. The predominant light can be directed from 
a point in the trough near the middle of the upper 
edge or near one of the corners of the painting, de- 
pending upon the requirements. This has been found 
very effective. The lighting of paintings depends 
also upon the hanging, which is too often done with 
a view toward keeping the bottom edges on a hori- 
zontal line instead of with a view toward placing 



COLOR PHENOMENA IN PAINTING 



293 




•5 




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§ 

I 

d 
o 

s 

I 

O 

ts 






294 COLOR AND ITS APPLICATIONS 

them in the proper position for lighting and observ- 
ing them. The wall covering is of importance and 
should be a dull, neutral, diffusing surface and pref- 
erably rather dark from a lighting viewpoint. This 
prevents undue annoyance from its image, as seen in 
the glass coverings of pictures on the opposite side 
of the room. The illusion produced by dark sur- 
roundings is striking, and there are many who advo- 
cate such wall coverings; however, others contend 
that the appearance of a gallery so hung is unaes- 
thetic. Much could be written on the daylighting 
of galleries. There have been some extensive studies 
of the problem made in various countries, but there 
is no general agreement so far as quality of light is 
concerned. Some advocate southern exposure, others 
a northern exposure. In general, artificial light is 
more readily controlled than daylight, and therefore 
lends itself more readily to obtaining proper effects. 

Inasmuch as paintings are very 
often poorly lighted, a simple illus- 



M . y\ tration of the geometrical principle 

-' / \ of lighting is shown in Fig. 121. 

N^ / V Every problem is readily solved 

K/"-, \ by such methods. In the design 

/ '^x\ o^ t^® lighting, both natural and 
/ ,^^ artificial, it is well to determine 

! ^^''' graphically the positions of the light 

sources and the expanse of sky- 
light (if the latter be diffusing), 
so that images of these bright 
— sources cannot be reflected from 



Fig. 121.— Illustrating the the glazed surfaces of paintings 

optics of picture lighting. .. . . ^ ^, 

directly into the eyes. 
72. Pigments. — A number of satisfactory pig- 
ments, among which are vermilion, indian-red, and 



COLOR PHENOMENA IN PAINTING 296 

the ochres, are derived from minerals; the animal 
kingdom supplies such pigments as carmine and sepia; 
and a large number of pigments, such as indigo, 
gamboge, and madder, are derived from the vegetable 
kingdom. Many of the aniline dyes are derived from 
coal tar. Many pigments are made artificially, such 
as ultramarine, cobalt-blue, zinc-white, Prussian-blue, 
chrome-green, and the lakes. The natural pigments 
derived from minerals are prepared by calcining and 
grinding and are purified by washing. For oil paint- 
ing these pigments are ground in such vehicles as 
linseed or poppy oil. For water colors the medium 
is usually gum water. The latter fixes the pigments 
on the surfaces to which they are applied and serves 
as a varnish. Such vehicles should preferably be 
colorless, because, for instance, the yellow color of 
linseed oil is likely to impart a greenish tinge to pale 
blue pigments. Turpentine is used as a thinner for 
oil paints. Varnish is employed to protect pigments 
from destructive agents usually present in the atmos- 
phere and from marring by abrasion. Oil varnishes 
are less liable to crack than spirit varnishes, and the 
quality of a varnish depends largely upon the resin 
of which it is composed. (See Chapter XVI.) 

There are three general classes of pigments used 
in paintings. The pastel pigments are quite destruct- 
ible. Water colors lend an airy delicacy to a paint- 
ing and iare quite appropriate in some classes of work. 
They are difficult to use, owing to their transparency 
and to the change in color that they undergo on 
drying. Oil colors, which according to some authori- 
ties were first used in canvas painting in about the 
year 1400, are the most durable — an important and 
necessary property of pigments for use in painting. 
Many pigments are permanent under moderate illumi- 



296 COLOR AND ITS APPLICATIONS 

nation when used alone, but there is always the danger 
of interaction between pigments when mixed. The 
permanency of the older paintings is no doubt due 
in part to the fact that the palette was rather poverty- 
stricken many years ago. Today there are several 
hundred pigments available, and therefore there is 
considerable danger of mixing pigments that interact. 
Anyone who has searched for pigments that are per- 
manent under excessive illumination and heat will 
perhaps readily agree that the permanency of pigments 
is only a matter of degree and that under severe 
conditions many so-called permanent pigments readily 
deteriorate. Gases and coal dust in the atmosphere 
and especially the products of the combustion of 
illuminating gas are known seriously to affect pig- 
ments. Light has a bleaching action and paintings 
often turn yellow when kept in the dark. A simple 
method of restoring paintings is to clean them with 
a cloth and set them in the sun for a day or two. 
This treatment, however, is rather severe for water 
colors and modern lake colors and is only satisfactory 
in some cases. Doubtless tests are being made con- 
tinually on the permanency of pigments, but there are 
few available data on the subject. In general min- 
eral colors are more stable than vegetable colors. 
Gases, moisture, interaction, heat, and light are the 
common causes of the deterioration of pigments. It 
has been found that most pigments are more per- 
manent in vacuo, protected from harmful gases and 
moisture. Some of the results of experiments indi- 
cate that the destructive rays in sunlight are chiefly 
the violet and ultra-violet rays ; that is, pigments have 
been found to deteriorate practically as quickly under 
blue glass as under clear glass. However, the most 
commonly used blue glass, namely cobalt-blue, trans- 



COLOR PHENOMENA IN PAINTING 297 

mits deep red and infra-red rays almost as freely as 

clear glass, so it is possible that heat was responsible 

for some of the deterioration. Of the great number 

of available pigments the following are found to be 

most durable: — 

Indian-red, largely ferric oxide; 

Venetian red, iron oxide; 

Burnt sienna, calcined raw sienna; 

Raw sienna, a clay containing ferric hydroxide; 

Yellow ochre, hydrated iron oxide; 

Emerald-green, a mixture or compound of copper 

arsenate and acetate; 
Terra verte, a natural green pigment found in Italy; 
Chromium oxide, green; 

Cobalt-blue, usually a mixture of the arsenate, phos- 
phate, or oxide of cobalt with alumina; 
Ultramarine ash, now made from soda, sulphur, 
charcoal, and kaolin. 

Pigments are far from spectral purity; that is, 
they reflect light of many wave-lengths. Interesting 
data obtained by Abney are shown in Table VI and 
spectrophotometric analyses of a number of these 
pigments, including those just described as being 
quite permanent, are shown in Figs. 122 and 123. 
It is seen that the mixing of colors is complicated 
owing to the complexity of the spectral character of 
the light reflected by pigments. For the sake of 
clearness it will be noted that the reflection curve 
of a neutral tint (white or gray) surface would be a 
straight line parallel to the base line in the last two 
illustrations. The colorist should be somewhat fa- 
miliar with the spectral characteristics of his pigments, 
because such knowledge is very useful in mixing pig- 
ments. The production of different hues by mixing 
pigments is possible because pigment colors are not 



298 



COLOR AND ITS APPLICATIONS 



monochromatic, that is, not of spectral purity. For 
instance, suppose monochromatic pigments were avaii- 



90 



a -Ye I low Ochre 

b- Co bait Blue 

c-Chromous Oxide g~ French Ultramarine 

d- Antwerp Blue h- Emerald Green 



e- Indigo 
f- Terre Verte 




040 



0.44 



04& 0.52 0.66 
M', VIML LEMGTH 



0.60 



0.64 



0.6& 



100 

90 

80 

70 

^60 

I 50 

UJ 

20 

10 

Q 



Fig. 122. — spectral analyses of pigments. 

i -Mercuric /odide _ ^ ^ • v // 
J -Vermillion ^m- Cadmiumyellow 
k - Gamboge ^ ' Incli^n Red 

e - Indian Velio w o- Carmine 



















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Fig. 123. — spectral analyses of pigments. 



0.65 



able and yellow and blue were chosen for mixing. 
Instead of obtaining green from the mixture, black 



COLOR PHENOMENA IN PAINTING 



299 



would be obtained, because the light transmitted by 
pure yellow flakes would be a monochromatic yellow 
which would not be transmitted by pure blue flakes; 
thus by the combination no light would be trans- 
mitted. One virtue of the poverty-stricken palette 
is the scanty possibility of the interaction of pigments; 
however, such a palette cannot be the source of a 
large variety of highly luminous and pure colors. 

Where high brightness and full saturation are 
desired it is well to avoid the production of the de- 
sired hue by mixture, as far as possible. This can 




G Y R 

WAVE LEIiOTH 

Fig. 124. — Illustrating the effect of the amount of the green components in blue 
and yellow pigments on the amount of ' black * in the mixtures. 

be illustrated by means of the mixture of blue and 
yellow. When these pigments are pure their mix- 
ture must result in the production of black. For 
instance, suppose the two pigments transmit light 
rays respectively as shown diagrammatically by Bi 
and Yi in Fig. 124. Neither pigment transmits green 
rays nor does one pigment transmit any rays that are 
transmitted by the other; therefore the resultant 
transmission will be zero and * black' results. If the 
so-called blue and yellow pigments are less pure, 
they may be found to transmit some green rays. 
These may be represented diagrammatically as Bz 
and Y2 in Fig. 124. It is seen that the resulting 
mixture of these two pigments will be a green of rela- 



300 



COLOR AND ITS APPLICATIONS 



tively low brightness corresponding to a green to 
which * black' pigment has been added. The greater 
the proportions of green rays transmitted or reflected 
by the two pigments, the less * black' will be present 
in the green resulting from their mixture, or, more 
correctly, the brighter the resultant mixture will be. 
Obviously, if the two components are selected suc- 
cessively closer and closer to green, finally the limit- 



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SUM OFTHt OREEM COMPONENTS INTHt BLUE AMD YELLOW 
PIGMEMTS 

Fig. 126. — Diagrammatic illustration of the results of mixing blue and green 
pigments containing various amounts of green. 



ing case would be that in which both components 
were green, G, and their mixture of course would 
produce green. The results of such a theoretical 
procedure are shown diagrammatically in Fig. 125, 
where the sum of the green components in the blue 
and yellow pigments is assumed to vary from zero 
to 100 per cent; meanwhile the amount of green in 
the mixture varies from zero to 100 per cent, and the 
amount of * black' from 100 per cent to zero. This 
simple diagram illustrates a very important point in 



COLOR PHENOMENA IN PAINTING 301 

the mixture of pigments. For the reason that the 
subtractive method of color-mixture always tends 
toward the production of * black' it is well to have 
available a large number of fundamental pigments 
representing as many hues as possible. This is 
especially desirable in the production of effects de- 
scribed in Chapter XII, Before the advent of modem 
art such a stock of pigments was not essential be- 
cause the tendency in the past was not to employ 
colors as pure as those found in common use today. 
A study of the curves in Figs. 122 and 123 is recom- 
mended in connection with the discussion presented 
in connection with Figs. 124 and 125 in order to 
obtain an idea of the relative brightnesses of various 
mixtures. See Chapter XVII, 

REFERENCES 

1. M. LucMesh, Light and Art, Lighting Jour. (U. S.), March, 1913. 

Light in Art, International Studio, April, 1914. 

The Lighting of Paintings, Lond. Ilium. Engr. March, 1914. 
Lighting Jour. (U. S.), April, 1914. 

The Importance of Direction, Quality, and Distribution of 
Light, Proc Amer Gas. Inst. 1913, 8, part 1, p. 783. 
Ostwald, Letters to a Painter. 
C. Martel, Materials Used in Painting. 
E. N. Vanderpoel, Color Problems, 1903 
C. J. Jorgensen, The Mastery of Color, 1906. 



CHAPTER XIV 
COLOR MATCHING 

73. Nearly all the phenomena influencing the 
appearance of colored objects have been treated else- 
where, but inasmuch as color-matching is a special 
art and is also of interest to everyone at times, a 
summary of those factors that influence the appear- 
ance of colors may not be out of place. The expert 
colorist is fully aware of the influence upon the ap- 
pearance of a color of retinal fatigue or after-images, 
surrounding colors, difference in sensibility of various 
parts of the retina, the spectral character and intensity 
of the illuminant, the surface character of the fabric, 
the peculiarities of dyes, and other factors. Every- 
one has encountered difiiculties in distinguishing or 
matching colors. For instance it is difficult to dis- 
tinguish some blues under ordinary artificial light, 
owing to the relatively low intensity of these rays. 
Many colored objects that have appeared pleasing 
in daylight are so changed under artificial light as to 
be quite unsatisfactory. Usually under artificial light 
the dominant hue of most colors shifts toward the 
longer wave-lengths. For instance, some purples 
will appear quite red under artificial light and bluish 
under daylight (Fig. 80). Such an example is methyl- 
violet. 

The surface character of a fabric plays an im- 
portant part in the appearance of the color. A col- 
ored fabric is ordinarily seen by reflected light, the 
light falling upon it being robbed of some of its rays 

302 



COLOR MATCHING 303 



by the selective absorption of the dye. If the surface 
is porous like wool, the light can penetrate deeply 
and will therefore suffer more internal reflections 
(# 64), finally reaching the eye quite pure in color. The 
degree of transparency of the fiber also exerts an 
influence. It is seen that such dichroic dyes as 
methyl-violet, cyanine, and dilute solutions of rhoda- 
mine or eosine pink will be very much influenced 
by the surface character and composition of the fiber. 
Wool and silk fibers are transparent, but those of 
cotton are not, hence light cannot penetrate as far 
into the latter as into silk or wool. Therefore, when 
these three fabrics are dyed in the same solution of 
a dichroic dye such as methyl-violet the cotton will 
appear bluer than the other fabrics. The finish of 
the surface is also of importance, because of the re- 
flection of unchanged light which dilutes the colored 
light of the fabric. Many aniline dyes exhibit the 
property of fluorescence, which alters the appearance 
of the fabric under different illuminants. A fabric 
colored with such a dye will appear differently at 
grazing incidence than when viewed normally to the 
surface. The actual distribution of light is of im- 
portance for the last two reasons. 

74. The lUuminant. — Inasmuch as the appear- 
ance of a color is so influenced by its environment, 
the question might be asked. Under what conditions 
is its appearance considered standard? Daylight has 
always been the accepted standard, because the arts 
have developed under daylight. Furthermore day- 
light of a certain quality is considered white light; 
that is, it has no dominant hue. Such an illuminant 
is logically a better standard than an illuminant which 
of itself will impart a definite hue to the colored fabric. 
The color of an illuminant (the color of a white sur- 



304 COLOR AND ITS APPLICATIONS 

face) is largely a matter of judgment which is influ- 
enced by many factors, and, inasmuch as daylight 
is quite variable, there has been a lack of agreement 
as to a standard daylight. Some have taken a white 
mist as representing such a standard, others have 
insisted upon the adoption of clear noon sunlight, and 
some have advocated the integral light from the sky 
and noon sunlight. Nevertheless a great many col- 
orists have adopted north skylight for color-matching. 

The difference between sunlight and skylight is 
demonstrated on viewing objects in the sunlight. 
Colors receiving direct sunlight appear * warmer,' and 
the shadows which receive only light from the blue 
sky appear of colder hues, although this comparison 
is not wholly justifiable, because of the difference in 
intensity. Of course the relative intensities of sun- 
light and skylight vary considerably, but on a clear 
day a shadow on a white blotting paper, which re- 
ceives light from the unobstructed blue sky, is only 
one-fifth or one-sixth as bright as the portion of the 
paper receiving both direct sunlight and skylight. 
The color of daylight varies throughout the day (# 62). 
Many colorists favor the use of daylight in the fore- 
noon, although the morning light is often of a pinkish 
tint. On cloudy dark days a purplish tint is often 
quite noticeable. Anyone engaged in accurate color 
discrimination is aware of the continual changes in the 
spectral character of daylight. Smoke and dust also 
alter daylight toward a reddish hue, and it is likely 
that the conditions in the upper stratum of the atmos- 
phere vary from time to time, producing a variation 
in the character and amount of scattered sunlight. 
The influence of colored surroundings in altering the 
color of daylight is very important. Clouds, adjacent 
buildings, green foliage, and the color of interior 



COLOR MATCHING 305 

walls exert a very noticeable influence on the color 
of daylight. Perhaps the most annoying feature of 
daylight is its unreliableness. In some climates the 
actual hours that a satisfactory daylight is available 
for color-matching are few. In congested and smoky 
cities this useful period is further reduced, so that 
there has always been a demand for an * artificial 
daylight.' Satisfactory units of this kind are now 
available (#62), since the advent of highly efficient 
steady light sources such as the gas-filled tungsten 
lamp. It is of course impractical to furnish an arti- 
ficial daylight to match the different kinds of daylight, 
to which various colorists have become accustomed. 
It is too much to ask any manufacturer to supply 
artificial daylight which will exactly match daylight, 
altered by reflection from adjacent colored objects, 
which will be different in most cases. If, however, an 
artificial daylight is available to fill the demand of the 
colorist, he should be willing to compromise and give 
the unit a fair test. If it differs slightly from the 
daylight to which he is accustomed, yet shows no 
peculiar spectral characteristics, the colorist can 
readily adapt himself to the slightly altered condi- 
tions. If the artificial daylight is composed of inde- 
structible color-screens the colorist can be assured 
that he has an invariable standard that will serve 
him twenty-four hours a day — a most desirable char- 
acteristic. There is some advantage in the produc- 
tion of a colored glass screen for use with tungsten 
lamps because the quality of light can be varied b*e- 
tween sunlight to blue skylight by varying the tem- 
perature of the lamp filament. As was shown in 
# 62 the glass developed by the author alters the light 
from the vacuum tungsten lamp operating at 7.9 
lumens per watt to noon sunlight quality; howeverj 



306 COLOR AND ITS APPLICATIONS 

when used with the gas-filled lamp operating at about 
16 lumens per watt, artificial skylight is produced. 
This is a very convenient method of utilizing the 
same glass to produce artificial daylight of different 
kinds. Spectrophotometric tests afford the only thor- 
ough means of analyzing an artificial daylight. Di- 
chroic dyes and some mixtures of aniline dyes are 
greatly influenced by the spectral character of the 
illuminant and therefore afford a ready means for 
determining approximately the satisfactoriness of an 
illuminant for color-matching purposes. Such mix- 
tures can be readily made so that fabrics dyed with 
them will appear of the skme hue under a certain 
illuminant, yet under another illuminant they will 
appear quite unlike. The two dyes used in screens 
c and d, Fig. 17, are examples of this character. 
Mixtures that appear green under daylight but quite 
different under another illuminant can be readily 
made by mixing naphthol-yellow with acid-violet and 
an orange with a deep bluish aniline dye. Two blue 
dyes can be readily made to appear practically alike 
under daylight, one consisting of a rather pure blue 
and the other having the common characteristic of 
transmitting deep red rays. Under an artificial illu- 
minant, rich in red rays, the latter will appear quite 
reddish as compared to the former. A weak solu- 
tion of erythrosine or rhodamine when added to a 
weak solution of potassium bichromate will produce 
a yellow in artificial light; however, in daylight it will 
appear quite pink. Such combinations can be readily 
produced by examining the spectra of the dyes and 
by combining them judiciously. Excellent dichroic 
dyes are methyl-violet and cyanine. These striking 
instances of the effect of the illuminant are well 
known to dyers and other colorists. 



COLOR MATCHING 307 

75. The Examination of Colors. — In examining 
colors it is well to understand the peculiarities of 
vision. The fovea centralis of the retina, where 
vision is most acute, is directly opposite the middle 
of the pupillary aperture. A small area around this 
point has been named the macula lutea. The center 
of this region, which is called the * yellow spot,' owing 
to its color, often manifests itself in the examination 
of colors (#65). It apparently absorbs blue rays 
somewhat, and its effect is quite noticeable on view- 
ing bright colors. The effect is particularly notice- 
able roughly outlined in after-images produced by 
large bright colored areas. Bright colors are difficult 
to examine, owing to retinal fatigue and to the promi- 
nence of after-images and successive contrast. This 
annoyance can be reduced by decreasing the intensity 
of illumination or by the use of neutral tint glasses. 
These methods are perhaps questionable, but are 
certainly less objectionable than the use of blue- 
green glass, as is used by some for the examination 
of bright red and orange colors. Paterson^ recom- 
mends the use of a gelatine film dyed with malachite 
green (a blue-green) for the examination of such 
highly luminous colors. A practically neutral tint 
screen can be made of a solution of nigrosine in 
gelatine. 

The effect of simultaneous contrast is often very 
great, for colors are apparently altered in hue and 
brightness by the influence of an adjacent color 
(Plate III). A black pattern on a red ground will 
appear of a blue-green tint. A white surrounded by 
green will appear brighter and of a pinkish tint. 
ChevreuP published an extensive work on this 
subject many years ago which goes into elaborate 
detail concerning contrast. In order to examine the 



308 COLOR AND ITS APPLICATIONS 

color of a thread or portion of a variegated color pat- 
tern, it is well to isolate the portion to be examined 
by means of a gray mask. The effect of contrast is 
so great that a colored thread may appear rich and 
pure in one pattern, yet quite dull amid other sur- 
roundings. This effect cannot be overlooked without 
inviting trouble in color-matching. 

If it were not for the effects of fluorescence, color- 
matching glasses could be used which have been 
especially adapted to the artificial illuminant. How- 
ever, as many of the aniline dyes fluoresce they 
should receive light of the standard daylight quality 
while being examined. This would not be the case 
with the combined use of an artificial illuminant and 
correcting spectacles. Of course the intensity of the 
artificial light must be several times greater than 
ordinarily required for seeing in order to compensate 
for the unavoidable absorption of the color-matching 
spectacles. This scheme is not new, for it has been 
practised in special cases by many expert col- 
orists. 

Colored fabrics are examined both by transmitted 
and reflected light. Colors are usually viewed by 
reflected light, the change in the color of the incident 
light being due to selective absorption. In a loose 
fabric of porous surface the light penetrates more 
deeply and is colored by many multiple reflections. 
As already stated silk and wool fibers are more trans- 
parent than cotton, and therefore permit a deeper 
penetration of the light. This means a greater num- 
ber of multiple reflections, and, for example, as in the 
.case of a dichroic dye, it results in a color corre- 
sponding to that which would be obtained with a 
cotton fabric dyed in a denser solution of this dye. 
The luster of silk is attributed to the smoothness of 



COLOR MATCHING 309 

the fibers. In examining colors by reflected light 
the distribution of the incident light is of importance 
inasmuch as some of the regularly reflected light is 
but slightly changed, owing to the fact that it does 
not penetrate the fabric. This tends to dilute the 
colored light and to make it appear less saturated. 
In installing artificial daylight it is well to distribute 
the light in a manner found satisfactory in day- 
light. 

Many aniline dyes in solid form reflect light 
complementary in color to that which they transmit. 
Crystals of some purple dyes appear green by reflected 
light. If the crystal be ground into a fine powder, 
the latter appears purple in color, because the light 
penetrates it and by transmission and multiple re- 
flections appears different than by specular reflec- 
tion. A borax bead containing cobalt may appear 
almost black, but when ground into a powder it ap- 
pears blue. Pigments when in a dense homogeneous 
mass are quite opaque and reflect light selectively. 
The phenomena of surface color is intimately related 
to the coefficient of absorption and the refractive 
index of the substance. Inasmuch as the phe- 
nomenon is not of sufficient iinportance to go into 
details regarding it, the reader is referred to any 
standard text-book in physics for an analysis of the 
phenomenon. Some fabrics exhibit changeable colors 
owing to their nap, which ends in a certain direction. 
If it ends toward the light, the latter penetrates the 
fabric to a considerable depth and is deeply colored 
by multiple reflections. If the nap ends away from 
the direction of the light, there is more specular reflec- 
tion and therefore less penetration, which results in 
a smaller change in color. 

The fibers of a fabric may be considered to hold 



310 COLOR AND ITS APPLICATIONS 

the dye in a state of suspension or solution, and there- 
fore fabrics are sometimes examined by transmitted 
light. In a special case of this kind of examination 
the fabric is held between the eye and the light and 
is viewed at a grazing angle. This is sometimes 
called the overhand method. By thus looking through 
the fibers, hues can be distinguished that are quite 
imperceptible in an examination by reflected light. 
This method is especially applicable to the examina- 
tion of colors of the darker shades. The rich appear- 
ance of these dark colors viewed in this manner is 
very striking. 

The change in color that a dyed fabric undergoes 
on drying is of great importance and often quite 
annoying. This need not be treated here, because 
the novice will learn very quickly that dyes in solu- 
tion and freshly dyed fabrics often undergo great 
changes in color. It is a point to be considered in 
color-matching. A great many dyes exhibit the prop- 
erty of fluorescence, among which are the eosines, 
phloxine, rhodamine, uranine, fluorescein, rose bengal, 
naphthalin-red, resorcin-blue, and chlorophyl. In 
matching strongly fluorescent colors it is seen that 
there is quite a difference in hue between the re- 
flected and transmitted light. For instance, by 
reflected light an eosine pink will appear redder than 
by transmitted light by the overhand method. This 
is due to the fact that the reddish fluorescence is 
most predominant in the light reaching the eye when 
the fabric is examined by the ordinary method of 
reflected light. By the overhand method this fabric 
will appear decidedly bluer, owing to the fact that the 
fluorescent light does not reach the eye in appreciable 
amounts. This effect may readily be demonstrated 
by dyeing two fabrics respectively by fluorescent and 



COLOR MATCHING 311 



non-fluorescent dyes so that they match by reflected 
light. They will be found to appear different by the 
overhand method. 

REFERENCES 

1. David Paterson, Colour Matching on Textiles, London, 1901. 

2. M. E. Chevreul, Principles of the Harmony and Contrast of 
Colours. 



CHAPTER XV 
THE ART OF MOBILE COLOR 

76. This subject will be treated from two view- 
points: first as to the relation of colors and sounds, 
and second, from the viewpoint of an art of mobile 
color independent of any other art. The treatment 
from the first viewpoint is not entirely one of choice. 
In fact one feels compelled to discuss the possibility 
and justification of such a relation because in the 
few instances that colors have been related to sound 
music the superficiality has been quite apparent. It 
took centuries of scientific study and analysis to 
mould musical chaos into a uniform art of measured 
music, and even today there are composers who are 
not reconciled to the generally accepted state of 
affairs. Even with this example of slow evolution 
in sound music before them, there have been a few 
who have had the temerity to relate colors and music 
before the public notwithstanding the meager data 
available. It is significant that the names of these 
* inventors' are not found among the experimental 
psychologists and other investigators who are un- 
earthing information that may some day form the 
foundation of an art of mobile color. 

Rimington, in a book entitled * Colour-Music,' re- 
peatedly compares colors and sounds, owing to the 
fact that both *are due to vibrations which stimulate 
the optic and aural nerve respectively.' He further 
states that *this in itself is remarkable as showing 
the similarity of the action of sound and color upon 

3" 



THE ART OF MOBILE COLOR 313 

US.' He presents other 'similarities' but in fairness it 
should be noted that he states that too much weight 
should not be given to them. Nevertheless, owing 
to the repeated citations by Rimington of these * simi- 
larities' one concludes that they influence him con- 
siderably in developing his so-called 'color organ.' 
If no stronger reason for interest in the art of mobile 
color existed, space would not be given to a discus- 
sion of this subject, but there are indications that such 
an art is waiting to be evolved. Furthermore, the 
relation between sound and color forms such an in- 
significant part in the author's thoughts regarding 
color music, that space would not be given to such 
a discussion if it did not appear necessary to clarify 
the matter by dispelling some of the superficial 
ideas regarding such a relation and by pointing out 
the limitations of certain attempts to present such a 
combination. 

There is no physical relation between sounds and 
colors. Sounds are transmitted by waves in a mate- 
rial medium, as proved by many experiments. Light 
rays are supposed by many to be transmitted by a 
hypothetical medium called the ether, but scientists 
are divided in their opinions regarding the existence 
of an ether. Furthermore, the two kinds of wave 
motion that are used to represent sound and light 
waves are necessarily different, because the former 
cannot be polarized while the latter can be. Light 
waves pass through what we term a vacuum, but 
sound waves cannot. These few fundamental differ- 
ences are sufficient to illustrate the futility of any 
claims that sounds and colors are produced in similar 
ways. 

Next let us consider the respective perceiving 
organs. The ear is analytic, for a musical chord 



314 COLOR AND ITS APPLICATIONS 

can be analyzed into its components. This is not 
true of the eye. In other words, the eye is a syn- 
thetic instrument incapable of analyzing a color into 
its components. Many examples have been cited 
in previous chapters of colors that appeared identical 
to the eye, yet differed greatly in spectral character. 
This difference in the two organs must necessarily 
influence the choice of a fundamental mode of pro- 
ducing * color music' 

As already stated, it is noteworthy that those few 
persons who have actually written * color-music ' are 
not found among the large group contributing to the 
development of the science of experimental psychol- 
ogy or to sciences closely akin to it. The relation 
between colors and sound music, if any exists, some 
day will be revealed, but only through systematic 
experimentation by investigators well versed in phys- 
ics, physiology, and psychology. There is value in 
experiments directly relating colors and music, but 
certainly it is too early to experiment before the 
public. Such procedure jeopardizes the chance for 
ultimate success, but, fortunately, past exhibitions of 
this character will have been forgotten long before 
color-music evolves into a form in which it will be 
recognized ultimately. 

For some time the author has been interested 
in the subject of mobile color as a mode of expres- 
sion similar to the fine arts, and has therefore watched 
with interest some attempts in relating colors and 
music. This interest has been almost entirely in an 
art of mobile color independent of any other art, but, 
besides preliminary experiments bearing on the sub- 
ject, some experiments with colors and music have 
also been performed. These will be touched upon 
later. Recently a musical composition by A. Scria- 



THE ART OF MOBILE COLOR 



315 



bine entitled * Prometheus ' was rendered by a sym- 
phony orchestra with an accompaniment of colors 
according to the *Luce' part as written by the com- 
poser for the * Clavier a lumieres' (Fig. 126). No 
clue is found in the musical score regarding the colors 
represented by the notes in the *Luce' part, or the 



A.SCR1ABINE 

_PROMETHEE_ 
LE POEME DU FEU 

; POUR GR,\NI) ()RCHi;SI RK El' PIANO 
! AVHORdllK.inOl^URS 

KTCLWlhRALUMii-RliS 





Fig. 126. — The *Luce' part for the * Clavier k lumieres* in Scriabine's 
* Prometheus.' (Upper staff in each portion is the * Luce ' part.) 

manner in which a * colored chord' is to be played — 
whether by juxtaposition or by superposition. The 
latter point is of fundamental importance, inasmuch 
as the eye is not analytic and a mixture of the colors of 
a * color chord' results in only a single hue. Some of 
those responsible for the rendition of this music, with 
color accompaniment, had, at different times previous 
to the final presentation, accepted both the Rimington 
scale and Scriabine's code (the latter having been 
discovered later in a musical journal published at 
the time of a previous presentation of the same selec- 



316 COLOR AND ITS APPLICATIONS 

tion in London) as being properly related to the 
music. The acceptance of the Rimington scale, in 
the absence of Scriabine's code, as being adapted to 
the music, and the final acceptance of the latter code, 
which was used in the public presentation, shows 
that at the present time there is no definite relation 
between colors and sound music, even in the minds 
of artistic interpreters of music. It must not be 
assumed that the colors in Table XXI bear any abso- 
lute relation to the corresponding musical notes. 
Rimington's scale apparently was chosen arbitrarily, 
as shown, merely for convenience in writing a * color 
score.' This is probably true of Scriabine's scale. 
Those familiar with the science of color would hardly 
consider it probable that a composer of sound music 
would hold the key to * color music' when they 
freely acknowledge their helplessness in definitely 
relating colors and musical sounds. Everything 
pointed to failure, and if one may judge from 
the criticisms of the rendition of * Prometheus' with 
the accompaniment of colors, after allowing for 
a considerable degree of conservatism and inertia, 
the relation of the colors and musical sounds 
was indefinite, unsatisfactory, and distracting. Con- 
sidering that the experimental work has not yet 
been done which should form a basis for expres- 
sion and arousing emotion by means of colors, no 
other outcome of superficially relating colors to 
sound music could have been expected. Even 
though this be an extremely progressive age, it 
is not likely that color music can evolve, in an 
acceptable form, from the imagination of a few 
persons. 

77. While it appears that the art of mobile color 
must evolve from fundamental experimental data 



THE ART OF MOBILE COLOR 



317 



TABLE XXI 
Color Codes 



Rimington 


Scriabine 


C Deep red 


Red 


C# Crimson 


Violet 


D Orange-crimson 


YeUow 


D# Orange 


Glint of steel 


E YeUow 


. Pearly blue and shimmer of moonshine 


F Yellow-green 


Dark red 


F# Green 


Bright blue 


G Bluish green 


Rosy orange 


G# Blue-green 


Purple 


A Indigo 


Green 


A# Deep blue 


Glint of steel 


B Violet 


Pearly blue and shimmer of moonshine 


C Invisible 





on the 'emotive value' of colors, of simultaneous 
and successive contrasts in brightness and hue, of 
sequences in hues, tints, and shades, of rhythm, etc., 
it is interesting also to experiment with colors in rela- 
tion to music. However, a safe elementary procedure 
in the latter experiments is to use colored light merely 
to provide the * atmosphere ' and gradually to intro- 
duce the element of varied intensity and, possibly, 
rhythm. Certainly it is far less presumptuous to use 
color in this manner in the absence of experimental 
data than to attempt to play a *tune' in colors as a 
part of a musical score. If it is only a matter of 
individual taste, any procedure is, perhaps, legitimate, 
but when the object is to develop an art of mobile 
color only cautious procedure is commendable. In 
providing * atmosphere' for a particular motif such 
superficial associational relations as blue-green for 
rippling water and red for fire (because artists paint 



318 COLOR AND ITS APPLICATIONS 

them thus) are insufficient. It is the deeper emo- 
tional relation that is desired which, perhaps, cannot 
be determined with certainty without many careful 
experiments on a large number of subjects. 

In developing an independent art of mobile color, 
what procedure shall be adopted? Certainly the 
fundamental experiments will be found to lie largely 
in the realm of psychology. The aim of the modern 
artist is not totally unrelated to the subject, and a 
group, of such artists perhaps would form a most 
interested audience for such experiments. The new 
movement in the theater which is striving for har- 
mony in action, lighting, and setting is not wholly 
unrelated to the subject under consideration. In 
experimenting with colors for the purpose of devel- 
oping an art of mobile color it may be profitable and 
encouraging to study the evolution of sound music. 
In Baltzell's * History of Music' we read 

* When we think of music we have in mind an organization of 
musical sounds into something definite, something by design, not 
by chance, the product of the working of the human mind with 
musical soxmds and their effects upon the human sensibilities. 
So long as man accepted the various phenomena of musical soimds 
as isolated facts, there could be no art. But when he began to 
use them to minister to his pleasure and to study them and their 
effects, he began to form an art of music. The story of music is 
the record of a series of attempts on the part of man to make 
artistic use of the material which the ear accepts as capable of 
affording pleasure and as useful in expressing the , innermost 
feelings.' 

The leading principles in music are rhythm, melody, 
harmony, and tone quality, and in the execution of a 
musical composition dynamic contrast is an essential 
factor in expression. 

*For ages after the birth of music, rhythm and melody were 
the only real elements, rh3rthm being first recognized. Music 



THE ART OF MOBILE COLOR 319 

that lacks a clearly-defined rhythm does not move the masses. 
It was not tmtil harmony appeared that music was able to claim a 
position equal to that accorded to poetry, painting, sculpture, and 
architecture.' * These principles, rhythm, melody, and harmony, 
became, when couched in the forms of expression adopted by the 
great masters, what we call modern music, and the story is one 
of a development from extreme simplicity to the complexity 
illustrated in modem orchestral scores.' 

The lesson we gain from the foregoing is to proceed 
patiently. Sound music had an elementary begin- 
ning evolving into its present form only after many 
centuries of experiment. 

A thought that naturally comes to us is this: Is 
there anything in Nature that suggests color music? 
Perhaps scenes full of color are suggestive of * atmos- 
phere' colors for musical compositions. Perhaps if 
the cycle of appearances of such a scene throughout 
a day were compressed into a period of five minutes, 
it might suggest what a composition in color music 
would be. Being unaccustomed to thinking of color 
apart from form, perhaps such studies would be 
fruitful. Certainly at first, in thinking of color for 
color's sake alone, one has a fueling that all solid 
foundation has been removed from beneath him. 

When it comes to experimental work one feels 
that the foundation has been restored, but is appalled 
at the immensity of the work to be done. The avail- 
able psychological literature yields some interesting 
information. Some work on affection pertaining to 
colors has been done, and the studies of rhythm are 
very extensive; however, the work, which eventually 
will form a definite basis for developing an art of 
mobile color, has hardly been begun. The meager 
data in color preference partially described in #66 
were obtained as a beginning of an inquiry into some 
of the elementary impressions produced by colors. 



320 COLOR AND ITS APPLICATIONS 

It appears from this work, which supports conclusions 
arrived at by others, that in general saturated colors 
are more preferred than tints or shades, the latter 
perhaps being generally more preferred than tints. 
There is some evidence that subjects who are less 
capable of isolating the colors, that is, more inclined 
to associate them with other experiences, prefer the 
tints and shades or so-called * artistic' colors. Some 
study has been made of combinations of colors, but 
without definite results at the present time. Of 
course all the known principles of harmony and con- 
trast of colors are available for use by the pioneer 
in the art of mobile color. However, no application 
of these principles can be made until extensive ex- 
periments have been performed. The * emotive value ' 
of various hues, tints, and shades, of simultaneous 
and successive contrasts in hue and brightness, and 
of rhythmic sequences in hue and brightness must 
be determined. Bradford found that saturated colors 
were most preferred and that the admixture of small 
proportions of another color have a lowering effect 
upon the preference of a color. Regarded objectively, 
the pure colors were found first in the preference 
order while those which appear to be adulterated 
with another color, were placed last. Cohn had pre- 
viously claimed that increase in saturation tended 
to make a color more pleasing. Titchener obtains 
results of a similar nature with the majority of his 
subjects, who definitely reject tints and shades of 
colors in favor of the saturated colors. While a 
color may be most highly preferred among a large 
number of colors the * emotive value' of this color 
is perhaps rather low as compared with many other 
things. For instance a dark blue color may be dis- 
tinctly more preferred than any other color in a cer- 



THE ART OF MOBILE COLOR 321 

tain group, yet it can hardly be compared in emotive 
value to a song by one of our operatic artists. As 
Titchener states, when compared in pleasantness with 
a good dinner or the scent of a flower the color patch 
will seem practically indifferent. Of course results of 
impressions are only relative and there is perhaps 
sufficient emotive value in colors alone to afford 
pleasure when combined to form color music. How- 
ever, the foregoing point is of interest in combining 
colors and sound music. Certainly a * color instru- 
ment' cannot compete with a symphony orchestra, 
which leads to the tentative conclusion that color in 
such a relation should be subordinated to the role of 
merely providing * atmosphere.' A * color instrument' 
of definite form is conspicuous in its feebleness when 
in the midst of a symphony orchestra. It was sug- 
gested that the colors be used in the rendition of 
* Prometheus' by combining them on the whole 
background of the orchestra setting without any arbi- 
trary limits, thus providing the atmosphere. The use of 
diaphanous curtains, draped in loose folds and per- 
haps kept moving gently by electric fans placed at a 
considerable distance, was recommended. However, 
neither of these suggestions was adopted, the colors 
having been played on a relatively small white screen. 
78. The mechanical construction of experimental 
apparatus for studying * color phrases' is simple. 
There are two general methods of procedure which 
immediately occur to the experimenter. In one, the 
various colors composing a * color chord' are separated 
physically by playing them on different parts of a 
white screen, thus introducing the factor of harmony 
and overcoming the lack of analytic ability of the 
visual apparatus. In the other the component colors 
of a color chord are mixed by superposition. Obvi- 



322 COLOR AND ITS APPLICATIONS 

ously, in the latter case harmony is limited to the 
presentation of colors successively and the predomi- 
nant factor in * composing' color music to be rendered 
by such an instrument would be that of color-mixture. 
In the former case the predominant factor would 
be that of the harmony of colors. In both proce- 
dures the element of rhythm and variation in bright- 
ness can be introduced. A decision regarding the 
mode of presenting colors — by juxtaposition or 
superposition — must be made before any serious 
attempts at composing color music can be made. 
Doubtless instruments employing both principles 
should be investigated, and with this in mind two 
simple instruments were constructed. One similar 
to that illustrated in Fig. 127 was used by Rimington, 
who employed arc lamps for sources of light. The 
various colors indicated in Table XXI were played in 
arbitrarily selected positions relative to each other. 
Obviously no purples appeared when the colors of the 
Rimington scale were played in this manner. This 
omission is inexcusable, for purple is of a definite hue 
and perhaps nearly as full of emotive value as any 
spectral color. The colors could also be mixed on a 
screen. A mechanical dimming apparatus was em- 
ployed for controlling the brightness of the colors. 
Rimington evidently has experimented considerably 
with such an apparatus, but gives little data that 
supplies fundamental information from which to 
develop an art of mobile color. Such an instrument 
was constructed by the author, using tungsten incan- 
descent lamps and fairly pure color filters, the wiring 
diagram being as shown in Fig. 127. Either mechani- 
cal or electrical dimmers may be used for control- 
ling the brightness of the colors. A similar instru- 
ment was used in the rendition of the *Luce' score 



THE ART OF MOBILE COLOR 



323 



in * Prometheus,' cited early in the present chapter. 
In order to overcome the arbitrariness of the relative 
positions at which the colors appeared upon the white 
reflecting screen an oscillating motion was given to 
the colors. By this means the colors never appeared 
completely superposed and appeared on various occa- 
sions on different parts of the screen. 



I 









^ «\^ vj 

S! ^ 5 

C^ ^ tii 



o ^ ^ 

"^ I ."^ 

5 Qi :^ 



















— r 


rti 


rh 






, 












J 




J 


1 


i 








J J 






S ifm 




^ 


\i^ 


LJJ 


Ur^ 




• m. 










1 — 1 



Esigsssssss^ 



^m^ 



Fig. 127. — Illustrating an instrument for studying the emotive or affective 
value of colors and color phrases; Rimington's color code is also shown. 

Another instrument constructed by the author, on 
the principle that any color can be matched by a 
mixture of three primary colors, namely red, green, 
and blue, is illustrated in Figs. 128 and 129. Red, 
green, blue, and clear tungsten lamps are respectively 
placed in series with specially constructed resistors. 
Each of the resistors, a, &, c, and d, contain ten mov- 
able contacts which are respectively connected to the 
corresponding keys on the keyboard. On pressing 
a given key the circuit is completed through the 



324 



COLOR AND ITS APPLICATIONS 



corresponding lamps and a certain amount of resist- 
ance wire. The line voltage is applied at F, and C 
is a common terminal for the four circuits. The 
clear tungsten lamps are in reality * daylight' lamps, 
thus producing white light. This light is used to 
dilute the colored light to any degree of saturation 
represented by the ten steps in intensity produced 




Fig. 128. — A color-mixture instrument for studying the emotive and affective 
value of colors and color phrases. 

by pressing the corresponding keys in the upper row 
marked W. Thus ten steps in intensity can be 
obtained for light of each primary color and white. 
Such a combination is, of course, arbitrary, but is 
sufficiently elaborate for preliminary experimental 
purposes. Hundreds of different colors are obtain- 
able, varying in brightness from that just perceptible 
to the maximum brightness, which is at the limit of 
comfortableness. The lamps are placed inside a 



THE ART OF MOBILE COLOR 



325 



velvet-lined box (Fig. 129) around the rectangular 
aperture. The colors are mixed by superposition 
and viewed at present on a circular white diffusing 
surface placed on the back of the box opposite the 
viewing aperture. The movable contacts are adjusted 
so that any corresponding set of three keys in the 
i?, G, and B rows will produce white light. Thus 
white light of ten degrees of brightness can be made 





B 
O 


G R G B 
O O O 












o 










B 


O O '"'' '^^ 
V G R B 






1 


/q 








e 


=,"" 


1 


1 







PROMT VIEW 



EHD-VIEW 



Fig. 129. — Showing the relative positions of the colored lamps in the apparatus 
diagrammatically shown in Fig. 128 

in this manner. The upper row of keys for producing 
white light has been installed in order to produce 
greater flexibility. 

Considerable personal experimenting has been 
done with these forms of apparatus, but little definite 
information has yet been derived. The foregoing has 
been presented to illustrate the procedure considered 
desirable in this work. The amount of experiment- 
ing that can be done with such apparatus is very ex- 
tensive, but the first question to decide concerns the 
character of the data desired. Many have dreamed 



326 COLOR AND ITS APPLICATIONS 

of color music, some have written about it, and a few 
have attempted to present it. The objects of this 
discussion have been to show that there is no art of 
mobile color at present; that meager constructive 
data exists concerning it; that there have been 
hardly more than superficial attempts made to present 
it; that psychological studies must be relied upon to 
point the way toward its development; that it is a 
field worthy of cultivation; and that there are defi- 
nite problems that must be studied in order to obtain 
foundation material for building up an art of mobile 
color. 

REFERENCES 

E. J. G. Bradford, On the Relation and Aesthetic Value of the 
Perceptive Types in Color Appreciation, Amer. Jour, of Psych. 1913, 
24, p. 545. 

J. Cohn, Gefiihlston und Sattigung der Farben, Phil. Stud. 1900, 
15, p. 279. 

D. R. Major, On the Affective Tone of Single Sense Impression, 
Amer. Jour. Psych. 1895, 6, p. 57. 

W. H. Winct, Color Preferences of School Children, Brit. Jour. 
Psych. 1909, 3, p. 42. 

L. R. Geissler, Experiments in Color Saturation, Amer. Jour, of 
Psych. 1913, 24, p. 171. 

E. B. Titchener, Experimental Psychology, New York, 1910, p. 149. 
A. W. Rimington, Colour-Music, London. 

C. A. Ruchmich, A Bibliography of Rhythm, Amer. Jour, of Psych. 
1913, 24, p. 508. 

G. H. Clutsam, The Harmonies of Scriabine, London Musical 
Times, March, 1913, p. 157. 

For a discussion of the rendition of * Prometheus' with an 
accompaniment of colors, see New York papers of March 22, 1915. 

J. D. MacDonald, Sounds and Colours, 1867. 

J. Aitken, On Harmony of Colour, Trans. Roy. Scot. Soc. of 
Arts IX, 1873. 

Mrs. E. J. Hughes, Harmonies of Tones and Colours, 1883. 

Arnold Ebet, Farbens3miphonie, Alleg. Musik Zeit. 1912, 39, 
Nos. 34 and 35. 

M. Luckiesh, The Language of Color, 1918. 



CHAPTER XVI 
COLORED MEDIA 

79. Available Coloring Materials. — In any kind 
of work a knowledge of the tools and materials avail- 
able is quite important. If one may judge from the 
questions that are asked by many interested in va- 
rious phases of color science, a brief outline of colored 
media and means of manipulating them should be of 
interest. The available coloring materials are very 
numerous, yet it is often difficult to find satisfactory 
pigments for a given purpose. It is of considerable 
advantage to have at hand a large variety of these 
materials; therefore a list of useful colored media are 
presented below. 

Colored glasses. — Sets of samples can be ob- 
tained from various supply houses. Signal glasses 
afford a limited number of fairly pure colors, usually 
red, yellow, green, blue-green, blue, and purple. 

Colored gelatines. — Very elaborate sets of colored 
gelatines can be obtained from theatrical supply 
houses. These are exceedingly useful, though lack- 
ing in permanency. If mounted between sheets of 
glass and kept in a ventilated position, many of them 
will be fairly durable. Complete sets of samples are 
very convenient. 

Colored lacquers. — Those intended for coloring 
incandescent lamps are very useful, although it is 
often desirable to mix these carefully in order to 
obtain colors of greater spectral purity. Such col- 
ored lacquers vary considerably in permanency, and 

327 



328 COLOR AND ITS APPLICATIONS 

wherever possible it is well to apply the coloring to 
sheets of glass which can be mounted at some dis- 
tance from the lamp. This insures a much greater 
permanency. 

Aniline Dyes. — For coarse work the cheap dyes 
used for coloring cloth will afford a fairly satisfactory 
range of hues. By judiciously mixing these dyes 
some fairly pure colors can often be obtained, al- 
though as mixing usually tends to produce muddy 
colors the better procedure is to have at hand a 
variety of fundamental pigments from which perhaps 
a satisfactory color can be selected. A variety of 
dyes of the better grade is almost indispensable for 
accurate color work. Such dyes are usually pure 
and fairly reproducible, and are the best coloring 
media for making photographic and other screens 
requiring pure colors. Sets of stains for tinting 
lantern slides are available. These coloring media 
can be purchased in various forms, liquid, powder, 
sheets, etc. It is probably surprising to the unini- 
tiated what a variety of coloring materials can be 
obtained in the stores of a city of moderate size. 

Artists^ Pigments. — Such pigments are classed 
as pastel, water colors, and oil paints. These all 
have their uses in color science. Water colors are 
now available in opaque moist pastes, which have 
advantages in some classes of work. 

Printers^ Inks. — Such a set of pigments will be 
found useful by those desiring to collect a variety of 
coloring media. They are especially adaptable to 
applications similar to those found in the print shop. 

Colored Papers. — The ordinary colored tissue 
papers are useful in demonstrating color effects, but 
in the study of the science of color no series is equal 
in purity and uniformity to the imported colored 



COLORED MEDIA 329 



papers, such as the Wundt colored papers supplied 
by Zimmerman which are mentioned in this work 
on various occasions. 

80. Pigments. — As stated in # 72 pigments are 
derived from mineral, animal, and vegetable matter, 
and in general the inorganic pigments are the most 
durable. The organic dyes are often more brilliant, 
and for a great many purposes are more satisfactory, 
than inorganic pigments, because the latter are usu- 
ally more opaque. The durability of pigments is a 
matter of degree, and depends upon the protection 
provided against moisture and other destructive 
agents in the atmosphere, such as gases and smoke. 
Few pigments will withstand excessive amounts of 
heat and light. An extended discussion of pigments 
is outside the scope of this chapter, but a few details 
regarding common pigments should prove helpful. 
The chemistry of pigments obviously is complex, so 
no simple rules can be formulated which will always 
guide the colorist in making mixtures of pigments 
that will not interact. 

Blue, — In general blue pigments reflect or trans- 
mit an appreciable amount of deep red rays, which 
becomes quite noticeable under ordinary artificial 
light. Ultramarine is considered by the artist to be 
a close approach to spectral blue in hue, yet it trans- 
mits a considerable proportion of red rays (Fig. 103, 
122). Natural ultramarine is obtained from a min- 
eral, but owing to its scarcity an imitation has been 
produced artificially in various grades. The artificial 
ultramarine is quite permanent and is insoluble in 
water, alcohol, turpentine, and oil. Ultramarine ash 
is a blue-gray pigment derived as a by-product in 
the preparation of natiiral ultramarine. 

Cobalt-blue is readily prepared quite pure and is 



330 COLOR AND ITS APPLICATIONS 

very durable, although it is far from a pure blue. 
Its reddish appearance under artificial light indicates 
that it reflects a large proportion of deep red rays, 
which conclusion is supported on analyzing the re- 
flected light (see 6, Fig, 122). Smalte is a powdered 
cobalt glass of a brilliant, transparent color that is 
quite durable. 

Prussian blue, like most artificial pigments, varies 
in quality. It is not generally as durable as the pre- 
ceding blue pigments, but is fairly permanent if used 
alone. It interacts with many pigments. In oxalic 
acid it forms a satisfactory writing fluid. It can be 
made by mixing a ferric salt with potassium ferro- 
cyanide. It can be deposited intimately in contact 
with a fabric if the latter be dipped first into one 
solution and then into the other. An excess of the 
potassium ferrocyanide forms a compound known as 
soluble Prussian blue. 

Indigo is derived from the vegetable kingdom 
and belongs to the lakes. It is insoluble in water, 
ether, oils, and cold alcohol. It dissolves in boiling 
concentrated alcohol and fuming sulphuric acid. In 
the latter solvent it forms saxon blue. 

There are numerous blue aniline dyes, but most 
of them transmit red rays as well as blue. 

Green. — Chromium oxide is a durable, opaque, 
deep green pigment. 

Emerald-green usually is a carbonate of copper 
mixed with alumina. It is quite opaque and durable 
and of a brilliant green color. 

Many greens are made by mixtures of such pig- 
ments as chrome-yellow and prussian blue, but the 
luminosity of such a mixture depends upon the 
amounts of green in the two pigments (#72). 

Terra verte, a native mineral found in various 



COLORED MEDIA 331 



parts of Europe, is opaque and durable and quite 
satisfactory in color. 

Malachite green is a natural carbonate of copper. 
It is pale green in color and moderately permanent. 
This pigment is being imitated artificially. 

There are several beautiful greens among the 
organic dyes, but of less durability. 

Yellow. — Gamboge closely represents spectral 
yellow. It is a gum resin employed very extensively 
in water colors. It is a bright, transparent, per- 
manent yellow. 

Cadmium-yellow is a brilliant, opaque pigment 
which forms fairly satisfactory greens by mixing 
with a number of the greenish blue pigments. It 
contains sulphur, and therefore should not be mixed 
with pigments containing lead. It is satisfactory in 
combination with zinc-white. 

Indian yellow is a permanent, fairly transparent, 
orange-yellow. The pure pigment burns easily, which 
is a means of detecting fraudulent adulteration or 
substitution. 

Chrome-yellow is lead chromate and varies in 
color from a lemon-yellow to a deep orange, depend- 
ing upon the chemical constitution and the admixture 
of other substances. It is used in both oil and water 
colors. 

Zinc chromate is an opaque, permanent yellow 
pigment which mixes well with other pigments. 

Potassium bichromate is a permanent yellow 
having many uses. It dissolves in water and appears 
a greenish yellow in slight concentration, but' ap- 
proaches a deep amber in a saturated solution. 

Satisfactory spectral yellows are rare, even among 
the large number of organic dyes available. Tart- 
razine, aurantia, martius-yellow, and naphthol-yellow 



332 COLOR AND ITS APPLICATIONS 

are representative of these dyes. They have a green- 
ish tinge. 

The ochres, which are earthy combinations of iron 
oxides, yield several yellow pigments. The native 
ochres are yellow and red. 

Red. — Carmine, which is obtained from the cochi- 
neal insect, closely imitates spectral red, and is con- 
sidered by many as the most beautiful red pigment 
known. It is opaque and mixes well with other pig- 
ments but is not very permanent. 

Vermilion is a natural compound of sulphur and 
mercury found in many places, and in mineralogy is 
called cinnabar. It is available in several hues, vary- 
ing from orange-red to deep red in both oil and 
water colors. 

Indian red and Venetian red are native ochres. 
Some of the yellow ochres are converted into light 
red pigments by calcining. 

The madder pigments, which are lakes, include 
various reds. The coloring matter is extracted from 
roots and united with alumina. These pigments 
are not very permanent. 

Lakes. — The coloring elements used in lakes are 
generally of vegetable origin. These possess the 
property of being precipitated from an aqueous solu- 
tion by metallic oxides, with which they combine. 
They have alumina, and sometimes other oxides 
associated with them, for a base to give them body. 
If it were not for the affinity of these oxides for many 
organic coloring matters many colors would not be 
available. For example, indian lake contains a col- 
oring matter extracted from lac; the coloring element 
in yellow lake is derived from berries; and the color- 
ing matters in cochineal and madder are extracted 
as described above. 



COLORED MEDIA 333 



White, — White pigments are used in diluting 
colored pigments for obtaining tints. 

White lead is carbonate of lead. It has many 
commercial names, but perhaps flake white is the 
most common. It is a very opaque pigment. Oil 
gives it a yellowish tint and should not be used very 
freely when a pure white surface is desired. White 
lead is attacked by sulphur and converted into black 
lead sulphide. It is more liable to react with other 
pigments than zinc-white, which is a formidable rival. 

Zinc-white is oxide of zinc. It possesses all of 
the good qualities of white lead and perhaps none 
of the objectionable features. It is claimed that the 
covering power of zinc-white is greater than that of 
white lead. Its sulphide is white, so that sulphur does 
not discolor it. 

Black. — Black pigments are usually carbons. 
Ivory-black is obtained from ivory waste and pos- 
sesses a rich black appearance. It produces excel- 
lent grays when mixed with white. Bone-black is a 
cheaper substitute. 

Lamp-black is obtained by burning certain sub- 
stances in an atmosphere containing little air or oxygen. 
Kerosene and coal gas yield soot which makes a satis- 
factory black. 

Nigrosine is a black pigment, soluble in water, 
which is very useful. It can be readily incorporated 
into various mediums and makes a fairly satisfactory 
neutral tint screen in a gelatine film. 

81. Solvents, — In making lacquers various sol- 
vents are available, the properties of some of them 
being given below. (See Table III.) 

Methyl alcohol (wood alcohol) mixes with water 
in all proportions. It is similar to grain alcohol as 
a solvent. 



334 COLOR AND ITS APPLICATIONS 

Ethyl alcohol (grain alcohol) dissolves many 
resins, oils, soaps, glycerol, camphor, celluloid, 
phenol, iodine, and many chlorides, iodides, bro- 
mides, and acetates. 

Acetone dissolves fats, oils, gums, resins, celluloid, 
and camphor, and mixes in ethyl alcohol and water. 

Ether, produced by distilling alcohol and sulphuric 
acid in proper proportions, dissolves fats, oils, resins, 
iodine, bromine, and many alkaloids. It mixes with 
alcohol, benzine, chloroform, and slightly with water. 

Amyl alcohol mixes with benzol, ether, alcohol, 
and slightly with water. It dissolves oils, camphor, 
resins, alkaloids, and iodine. 

Amyl acetate (artificial banana oil) mixes in all 
proportions with alcohol, amyl alcohol, and ether. 
It dissolves celluloid and is used in the preparation 
of collodion varnishes. 

Benzine should not be confused with benzene or 
benzol, the latter being derived from coal tar. It is 
a substitute for turpentine in paints, oils, and driers. 

Glacial acetic acid (pure acetic acid) mixes with 
water, alcohol, and ether. It dissolves oils, phenols, 
resins, and gelatine. 

Linseed oil is used as a vehicle in oil pigments. 
It dissolves hard resins, amber, and copal and is used 
for making varnishes. 

Poppy oil replaces linseed oil in oil pigments where 
the yellow color of the linseed oil is objectionable. 

Benzene is derived from coal tar. It mixes with 
alcohol, ether, petrolic ether, turpentine, and dissolves 
oils, fats, waxes, iodine, and rubber. It loosens paint. 
Benzol is an impure benzene. Toluol, toluene, and 
methyl-benzol are similar to it. 

Gelatine is soluble in hot water and concentrated 
acetic acid, forming, in the latter case, an adhesive 



I 



COLORED MEDIA 335 



paste. Potassium bichromate renders it insoluble 
on exposure to light. Formalin added to a warm 
aqueous solution and permitted to dry renders the 
gelatine insoluble in hot water. 

Turpentine dissolves fats, oils, and resins. It 
is used for thinning paints and varnishes. 

Venice turpentine is slowly soluble in absolute 
alcohol, but is readily soluble in ether, acetone, pe- 
trolic ether, benzol, and glacial acetic acid. It is 
used in fixing colors, in printing inks, and in spirit 
varnishes to give elasticity. 

Canada balsam is soluble in ether, chloroform, 
petrolic ether, benzol, turpentine, and gasoline. It 
is used to cement glasses, and owing to the fact that 
its refractive index is close to that of glass it practi- 
cally eliminates reflection and refraction of light at 
the surfaces in contact with it. For this reason it 
is excellent for cementing cover glasses on color 
filters. 

82. Varnishes. — A varnish is usually made by 
dissolving a resin in a medium such as alcohol, tur- 
pentine, or oil, the first forming a spirit varnish, the 
second a turpentine varnish, and the third an oil 
varnish. The so-called resins most commonly em- 
ployed are copal, sandarac, mastic, dammar, shellac, 
and amber. The properties of a varnish depend 
largely upon the resin and somewhat upon the solvent. 
If the solvent is volatile, like alcohol and turpentine, 
after the varnish dries the resin is left in the same 
state as before it was dissolved. These are quick 
drying varnishes. If the solvent be an oil, then both 
the oil and resin remain and the coating after drjring 
is pliable and tough. 

Copal is soluble in hot linseed oil; sandarac in 
alcohol; mastic in ether, in hot alcohol, and in tur- 



336 COLOR AND ITS APPLICATIONS 

pentine; dammar in alcohol and in turpentine; shel- 
lac in alcohol and in a solution of borax; amber in 
boiling linseed oil; gum arable in water forming a 
varnish for water colors; gum kauri in hot ether, in 
turpentine, in amyl alcohol, and in benzol; common 
resin in ether, alcohol, turpentine, benzol, acetone, 
or hot linseed oil. 

Common resin in wood alcohol forms a cheap 
varnish. An excellent spirit varnish is obtained by 
dissolving dammar in alcohol and turpentine, the 
proportions of the latter being respectively about four 
to one. A weather-proof varnish can be made of 
dried copal 7%, alcohol 15%, ether 77%, and tur- 
pentine 1%. 

83. Lacquers, — Shellac dissolved in alcohol and 
decanted after settling provides a cheap lacquer and 
solvent for some aniline dyes. Ordinary shellac is 
quite yellowish in color, so that the use of bleached 
shellac is sometimes advisable. The latter, however, 
does not dissolve as readily as the yellow shellac, 
but satisfactory proportions are one part of bleached 
shellac to eight parts of 90% alcohol. In making 
colored lacquers the aniline dyes are usually more 
satisfactory on account of their transparency, although 
they lack permanency when exposed to radiant en- 
ergy. The inorganic pigments are more opaque, but 
are usually more permanent. In general they do not 
dissolve, although they can be held in suspension. 
They are not as generally satisfactory as the aniline 
dyes for coloring media, excepting for their greater 
permanency. 

Photographers' ordinary collodion, which consists 
of pyroxylin (soluble guncotton) dissolved in ether 
and alcohol, can be used as a solvent for aniline dyes 
for coloring incandescent lamp bulbs. 



COLORED MEDIA 337 



Ordinary photographic film (from which the emul- 
sion has been removed) dissolved in amyl acetate, 
alcohol, or acetone, provides a satisfactory lacquer 
for lamp colorings. Ordinary clear celluloid scraps 
containing a large percentage of camphor are readily 
dissolved in acetone, thus providing a cheap lacquer 
for dyeing purposes. The latter celluloid scraps dis- 
solve in wood alcohol and in amyl acetate, but when 
the lacquer drys it becomes white; however, this 
provides a means of making a cheap though not very 
satisfactory opal lacquer. Ordinary white celluloid 
scraps dissolved in wood alcohol provide a very cheap 
opal lacquer. A permanent opal solution can be made 
by mixing pure zinc-white to a fair consistency, using 
but little oil with a few drops of gold size. This can 
be applied by stippling with a flat-headed brush. 
Obviously this solution can be readily colored by 
mineral pigments, but such mixtures are not very 
transparent. 

If it is desirable to make a transparent glass dif- 
fusing or translucent, a saturated solution of epsom 
salts in warm water is satisfactory. After applying 
this solution and permitting it to dry, a surface is 
obtained similar to that produced by etching or sand 
blasting. This can be colored with some of the dyes 
soluble in water. Such a surface is not permanent. 

84. Dyeing Gelatine Films. — Perhaps the most 
convenient manner of making color filters for a large 
variety of uses is in applying the coloring matter to 
gelatine. A simple scheme is found in placing, a 
photographic plate in an ordinary fixing solution for a 
few moments, and, after thoroughly washing it, per- 
mitting it to soak in an aqueous solution of the dye. 
The gelatine coating will absorb considerable of the 
dye, the depth of coloring being controlled chiefly by 



338 COLOR AND ITS APPLICATIONS 

the concentration of the colored solution and some- 
what by the period of time the plate is permitted to 
remain in the bath. If the coloring is too dense, some 
of it can be washed out by placing the plate in run- 
ning cold water. It is sometimes necessary to acidu- 
late the solution slightly or to add ammonia, alcohol, 
etc., in order completely to dissolve the dyes, but 
this does not usually interfere with the above process. 
Better control is obtained by adding an aqueous 
solution of the dye to a solution of gelatine in warm 
water and flowing the dyed gelatine on a level plate 
of glass or other transparent media. This procedure 
lends itself to accurate reproduction. It is advisable 
to use a harder variety of gelatine, which can be pur- 
chased from chemical supply houses. From four to 
six per cent of gelatine (by weight) in water is found 
satisfactory. The gelatine is permitted to soak in 
cold water for an hour or more ; then the vessel con- 
taining it is placed in a basin of water and gently 
heated. It is advisable not to heat the water above 
50 deg. C or more than is necessary to liquefy the 
gelatine. This solution should be filtered through 
a coarse cloth free from lint, and the plate should 
be flowed in a dust-free atmosphere. Sometimes 
it is well to warm the glass plate before flowing the 
gelatine. The amount of gelatine solution should be 
approximately one cubic centimeter to ten square 
centimeters of area. It is well to permit the plate 
to dry uniformly until completely hardened. The 
surface will not be optically plane, but where this is 
necessary another plate glass may be cemented on 
top of it with Canada balsam and a moderate pressure 
should be applied for several days. When dried at 
a temperature of 40 deg, C, only a day or two is re- 
quired for the balsam to harden. After the plates 



COLORED MEDIA 339 



I 



are thoroughly dry they can be bound together at 
the edges with metal strips or gummed paper. A dry- 
ing cabinet heated by means of carbon incandescent 
lamps is very safe and convenient, and the tempera- 
ture can be readily regulated by varying the number 
of lamps in operation. 

Gelatine sheets can be made by flowing the gela- 
tine solution upon a level aluminum plate, from which 
they are readily removed after drying. Doubtless 
there are better processes for the latter procedure 
used in the manufacture of such sheets. 

85. Celluloid. — This material is of interest be- 
cause of its use in lacquers and its transparency and 
durability, which make it a substitute for glass or 
gelatine films. An undesirable characteristic, how- 
ever, is its inflammability, although tests indicate 
that the commercial celluloid is not dangerously 
explosive. It resists most acids and bases of 
moderate concentration when cold. Glacial acetic 
acid rapidly dissolves it, and when this solution is 
poured into water the nitrocellulose, camphor, and 
other substances are precipitated. It dissolves in 
alcohol, the best solvent being camphorated alcohol 
(10 parts camphor to 100 parts alcohol). Acetone, 
either the liquid or vapor, dissolves it. Celluloid 
films can be made by casting or by a continuous 
process, and can be polished by felt disks or rollers, 
using powdered pumice stone, soap, or polishing 
oil. 

Celluloid takes up dye very well from a solution 
of the coloring in alcohol. The colors for staining 
should act like mordants, or their application should 
be similar; that is they should penetrate deeply into 
celluloid, thus coloring the mass. The usual solvents 
are alcohol, acetone, acetic acid, and amyl acetate. 



340 COLOR AND ITS APPLICATIONS 

In staining celluloid it is first moistened by a soften- 
ing agent in which the aniline dyes are mixed; then 
on dipping the celluloid into such a solution the dye 
penetrates the mass.* 

Celluloid is readily colored by the foregoing 
methods, but can also be colored by means of mineral 
dyes, though if transparency is desired, which is the 
condition considered most important here, these 
colorings are not as satisfactory, although they pro- 
vide permanent colors. A solution of indigo in sul- 
phuric acid and neutralized by potassium hydroxide 
produces a blue dye. Another method which fur- 
nishes a more satisfactory blue results in the pro- 
duction of Prussian blue. The celluloid is immersed 
in a bath of ferric chloride, and after drying is dipped 
into a bath of potassium ferrocyanide. To color 
celluloid green, it is dipped into a solution of verdigris 
and ammonium chloride. To color it yellow it is 
immersed in a solution of lead nitrate and then dipped 
into a solution of neutral potassium chromate. Solu- 
tions of chrysoidine, auramine, and many aniline dyes 
in alcohol are satisfactory. To color celluloid red it 
may be dipped first in a dilute solution of nitric acid, 
then immersed in an ammoniacal solution of carmine. 
Color will be readily absorbed by celluloid if its sur- 
face is first sandblasted. 

86. Phosphorescent materials. — A variety of 
phosphorescent materials are available from chemical 
supply houses in varying degrees of purity and of 
various colors. These have their place in colored 
effects, especially for demonstration purposes. They 
have been used in theatrical productions, but the 
greatest drawback is the difficulty of obtaining an 
illuminant emitting rays of short wave-lengths (which 
are the most effective in exciting phosphorescence) 



COLORED MEDIA 341 



in sufficient intensities. The bare carbon arc and 
the quartz mercury arc are the most intense excitants 
for this purpose among artificial light sources. Lumi- 
nous calcium sulphide, sometimes known as Balmain's 
paint, is cheap and active and emits phosphorescent 
light of fairly long duration. It forms the basis of 
several cheap though not highly satisfactory phos- 
phorescent paints. 

Phosphorescent oil paints can be made by using 
pure linseed oil instead of the varnish which is ordi- 
narily used in phosphorescent paints. For artists' 
paints the varnish should be replaced by pure poppy 
oil. Phosphorescent material can be applied to cloth 
and paper by omitting the varnish, mixing the powder 
in water, and applying this paste in a convenient 
manner. For applying to glass or porcelain, the 
varnish is replaced by Japanese wax in a slightly 
greater quantity, and olive oil is added. These mix- 
tures can be fired successftdly when air is excluded. 
Water glass (sodium silicate) is a satisfactory pro- 
tecting agent for such applications. 

87. Miscellaneous Notes. — For purely decorative 
effects of a temporary nature some of the colored 
metallic salts that crystallize when the solvent is 
evaporated are quite useful. For instance, if a sat- 
urated solution of potassium bichromate be added to 
a rather concentrated aqueous solution of gelatine, 
and this mixture be flowed while hot upon a level 
plate glass, on cooling it forms a yellow diffusing 
filter of crystalline structure. Such screens do not 
have a wide application; nevertheless they can be 
used for temporary decorative purposes. A weak 
solution of potassium bichromate can be used in 
gelatine without crystallizing or drying. The greenish 
tinge of this yellow can be overcome, if desirable. 



342 COLOR AND ITS APPLICATIONS 

by an addition of a slight quantity of a dilute solu- 
tion of a red or pink dye. 

The air brush is a useful instrument for the appli- 
cation of liquid colorings, especially when the pig- 
ments do not readily dissolve in lacquers. Lamps 
and other objects can be readily colored by immer- 
sion in a colored lacquer, but this is not a very satis- 
factory procedure when the coloring matter is merely 
held in suspension. By means of an air brush any 
colored solution can be readily applied to an object 
with a fair degree of uniformity. Perhaps the most 
discouraging factor in the production of colored 
lighting effects is the lack of permanent blue and 
blue-green pigments that will readily dissolve in a 
satisfactory lacquer. Prussian blue and cobalt-blue 
are quite permanent, but insoluble in common lac- 
quers. These can be successfully applied by means 
of an air brush when they are held in suspension in 
a lacquer of thin varnish. By occasionally diverting 
the flow of air through the liquid such insoluble pig- 
ments can be kept in suspension in the binding solu- 
tion. For this class of work a small motor operated 
from two or three dry cells or a small transformer and 
equipped with a vertical stirring rod is exceedingly 
useful. 

Pigments can be readily tested for durability by 
placing them on strips of glass and partially covering 
them with glass. These should be exposed to sun- 
light or to the radiation from an arc lamp, keeping 
part of the pigment covered. The exposed portions 
should include both the unprotected portion and that 
protected by the cover glass. Another convenient 
method, depending of course upon the final uses to 
which the pigments or lacquers are to be put, is found 
in applying them directly to incandescent lamp bulbs. 



COLORED MEDIA 343 



The lamps should be dyed in pairs, and one should 
be preserved while the other be operated on normal 
or slightly above normal voltage. If the pigments 
are eventually to be exposed to the weather, the tests 
should be made out of doors. 

These are a few data that have arisen in ex- 
perimental work in the study and application of the 
science of color and in the production of various color 
effects which may prove helpful to those interested in 
color. See next chapter. 

REFERENCES 

M. Toch, Materials for Permanent Painting, 1911; Chemistry 
and Technology of Mixed Paints. 

E. J. Parry and J. H. Coste, The Chemistry of Pigments. 

F. S. Hyde, Solvents, Oils, Gums, and Waxes. 

C. H. Hall, Chemistry of Paint and Paint Vehicles. 
W. R. Mott, Paint and Dye Testing, Trans. Amer. Electrochem. 
Soc. 1915. 



CHAPTER XVII 
CERTAIN PHYSICAL ASPECTS AND DATA 

88. A perusal of the literature on colored media and 
a general acquaintance with color industries has led to 
the conclusion that the chemistry of such substances 
greatly dominates the physics in color-technology. In 
fact, much of the physics of color is so little used in some 
of these activities that it is either not generally under- 
stood by color-technologists or its value is underestimated. 
Spectral analyses — the quantitative determinations of 
the spectral characteristics of colored materials — pro- 
vide the foundations for many important aspects of color- 
technology and without such data some work is con- 
ducted more or less blindly. With such data and those 
derived from less analytical methods, many interesting 
facts of color-technology can be bared and various 
factors can be determined which are unapproachable 
from the viewpoint of chemistry or from ordinary visual 
inspection. 

Of the various methods of analyzing color, that of 
the spectrophotometer is the most analytical and it 
provides data of far greater usefulness in the physics 
of color than the data which are yielded by any of 
the other methods. By this method the reflection- (or 
transmission-) factors of the coloring media are de- 
termined for radiant energy of all wave-lengths in the 
visible spectrum. When these are plotted we have the 
spectral reflection (or transmission) curves for the 
visible spectnun. By the same method the spectral 
character of an illuminant may be obtained. By multi- 

344 



i 



CERTAIN PHYSICAL ASPECTS AND DATA 345 

plying the relative energy-values of the various wave- 
lengths of any illuminant by the corresponding visi- 
bilities of radiation, the spectral luminosity-distribution 
curves are obtained for the given illuminant. These 
latter will vary with the illuminant and are often of 
greater importance than the spectral energy-distribution 
curves from a visual viewpoint. It is obvious that by 
multiplying corresponding spectral values, the spectral 
energy-distribution and luminosity-distribution curves 
of any colored medium may be readily obtained for any illu- 
minant. Such data and their uses will be presented later. 

Owing to the indefiniteness and limitations of the 
data 3aelded by most of these so-called -colorimetric 
methods and the difficulties attending the use of the 
monochromatic colorimeter at present, this chapter will 
be confined almost entirely to spectrophotometric data 
and their uses. Many instances arise when the degree 
of absorption for ultra-violet and infra-red rays is of 
interest. The former can be determined readily by 
spectrophotography and the latter by means of such 
energy-measuring instnunents as the bolometer or ther- 
mopile. 

89. Types of Colored Media. —Three classes of 
colored media will be represented and discussed, namely, 
pigments, dyes, and vitrifiable colors or colored glasses. 
Pigments are distinguished from dyes by their insolu- 
bility in their vehicle, while dyes are soluble. This 
distinction may appear arbitrary, especially in some cases, 
however, it is employed to some extent and is a con- 
venient classification. Pigments may be distinguished 
from paints in that the latter are pigments in a vehicle 
or medium. Vitrifiable colors are those which impart 
color to glass and to similar substances. Among pig- 
ments are found two general classes; one in which 
each particle is homogeneous and the other in which 



346 COLOR AND ITS APPLICATIONS 

a colorless base has been colored by depositing coloring 
matter upon it. Colored media vary in many physical 
characteristics such as opacity, fineness, and refractive- 
index, and they may be considered as varying in * color- 
ing power.' 

The color of a pigment in a finely divided state, 
whether the particles are separated by air or by a vehicle, 
is due to innumerable selective reflections from, and 
transmissions through, the minute particles. If the 
powdered pigment is given a smooth surface by pres- 
sure it does not appear as pure in color as when it is 
loosely packed because in the latter case a greater 
proportion of the incident radiant energy is able to 
penetrate more deeply into the body and becomes colored 
by selective reflections and transmissions. Radiant 
energy is regularly reflected from even the small sur- 
faces of the particles of pigment and in those cases where 
the minute areas of surface are properly oriented, this 
regularly reflected light does not find its way further 
into the pigment but is reflected practically unaltered 
in spectral character as compared to that energy which 
penetrates further into the mass. Thus, there is always 
reflected from pigments some radiant energy which is 
practically unchanged in spectral character which ac- 
counts partly for the general lack of purity of the colors 
of pigments. It is seen that the character of the surface 
is important. Furthermore, the refractive-indices of 
the pigment and of the vehicle (air in the case of dry 
powders) are of importance because the amount of light 
regularly reflected from a surface is dependent upon 
these refractive-indices. A careful study of the influence 
of the vehicle upon the color of a paint leads to inter- 
esting data from this viewpoint alone. 

Careful observation will reveal the influence of the 
porosity of a pigment-surface upon its color. An excel- 



CERTAIN PHYSICAL ASPECTS AND DATA 347 

lent example for the purpose of illustration here is the 
color of a white cotton fabric compared with that of 
a white silk fabric after both have been soaked in the 
same dye-solution. In Fig. 130 are shown reproductions 
of microphotographs of white cotton and silk fabrics 
as photographed against a black background. It is 
seen that the silk is more transparent than the cotton 
fibers; in fact, the cotton fibers are merely translucent 
as compared with the transparency of silk fibers. The 
latter permit the radiant energy to penetrate more 
deeply, in general, than the cotton fibers; in other 
words, the cotton fibers by diffuse reflection turn the 
energy backward before it has penetrated very deeply. 
For this reason the silk fabric appears of a purer color 
compared with that of the cotton fabric dyed in the 
same solution, the result, in the case of the silk, being 
similar to that which would have been obtained with the 
cotton if the latter had been dyed in a more concen- 
trated solution of the dye. 

In a manner similar to the case of pigments the 
solvent appears to have certain influences upon the color 
of a solution of a dye although this subject has not been 
thoroughly studied. The substance upon which a dye 
has been deposited by immersion is also of importance 
in spectral analysis as is indicated by the case of dyeing 
cotton and silk fibers. Fig. 130. The transmission- 
factor of a dye-solution is a simple logarithmic function 
of the depth of a given solution or of its concentration, 
but this relation varies with the wave-length, in general 
in no definite relation between wave-length and spectral 
transmission-factor. For this reason no simple relation 
between total transmission and depth or concentration 
can be established. Such values of total transmission 
must be determined by direct measurement or by in- 
tegration, as will be discussed later. 




Fig. 130. — Cotton. 





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Fig. 130. — Silk. 



CERTAIN PHYSICAL ASPECTS AND DATA 349 

Colored glasses can be treated much in the same 
manner as dye-solutions. A given concentration of 
coloring material in a glass, that is, a given colored 
glass, apparently obeys the same law relating thickness 
and transmission-factor for a given wave-length as a 
dye-solution. However, it is not established that the 
introduction of various amounts of the coloring material 
(generally metallic oxides) results in corresponding con- 
centration as would be true in the case of dyes. In glass 
there is more or less chemical action and the uncertain 
conditions of melting make this point difficult to 
decide. 

The physics of the process by which glasses are 
colored by means of metallic compoimds is not wholly 
clear. There are many chemical analogies which are 
of interest for their parallelism to the colors imparted 
to glasses by the metals in different states but the reasons 
for the appearance of the colors cannot be considered 
as being thoroughly established. Gamett^ has pre- 
sented a very interesting discussion of the colors ex- 
hibited by certain glasses in which metallic oxides had 
been incorporated. It is a common supposition that the 
colors of certain glasses, such as gold red glasses, are 
due to the presence of very minute particles of metal. 
Solutions of some metals exhibit colors which are often 
exhibited by colored glasses in which the same metals 
have been introduced. Siedentopf and Szigmondy, by 
powerfully illuminating specimens of colored glass and 
colored colloidal solutions of metals obliquely, or at 
right-angles to the line of sight, were able to detect the 
presence of the metallic particles. Gamett's work ex- 
plained some of their observations. 

It is commonly considered that metals color glass 
in two ways, one by being in a state of true solution in 
the glass and the other by being in a colloidal state. 



350 COLOR AND ITS APPLICATIONS 

An example of the former is copper blue-green glass and 
of the latter, gold red glass. 

In dealing with the physics of colored media from the 
viewpoint of the physicist, one cannot avoid the con- 
clusion that there is a wide application of physics to color- 
technology in many directions quite unexplored. 

90. Pigments. — In presenting data which it is hoped 
will be of direct use to others, only those colored media 
have been selected which are thought to be fairly con- 
stant in composition and representative. The spectral 
reflection-factors of a group of dry powdered pigments, 
commonly used in the paint industries and which from 
general observation appear representative, were de- 
termined by means of the spectrophotometer and the 
data are presented in Table XXII. The light was re- 
flected from a thick layer of the powder, the surface 
being gently smoothed by means of a sheet of plane 
glass. Whites and blacks have been omitted but these 
are by no means always neutral pigments. Whites are 
very commonly yellowish and blacks (which are only 
approximately black, varying in reflection-factor from 
0.02 to 0.1) are often bluish or reddish. Although these 
departures from neutrality are not relatively great they 
are sufficient to be detected by means of the spectropho- 
tometer. Such small departures are readily detected by 
painting the inner surface of a box with such a sup- 
posedly neutral pigment and by viewing a white surface 
indirectly lighted by means of a light-source inside the 
box. The visible radiation suffers innumerable re- 
flections (see # 65) from the walls of the box and that 
which illuminates the white surface is therefore much 
more colored than the pigment would appear under di- 
rect illumination. The spectral reflection-factors of pig- 
ments are more difficult to obtain than the transmission- 
factors of dyes in solution because in the former case 



CERTAIN PHYSICAL ASPECTS AND DATA 



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352 



COLOR AND ITS APPLICATIONS 



more or less energy is regularly reflected from the 
particles directly into the instrument. Care must be 
taken to avoid placing the pigment surface in such a 
position with respect to the slit of the instrument and 
to the light-source that an imdue amount of regularly 
reflected visible radiation enters the instrument. The 
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pigment. At any angle some of the energy is regularly 
reflected from the minute portions of the surfaces of 
the particles which are properly oriented. This accounts 
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in Figs. 131 and 132. 

Spectral analyses in the ultra-violet and infra-red 



CERTAIN PHYSICAL ASPECTS AND DATA 



353 



regions are often of interest in general color-technology. 
In the former case spectrophotography is the simplest 
method of attack although the procedure is a tedious 
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Fig. 132. — Pigments. 

various wave-lengths. Besides this the ordinary pre- 
cautions of photographic procedure must be taken. 
Another possible method is that which involves the use 
of the photo-electric cell. No systematic data on pig- 
ments in the ultra-violet region have been obtained 
so none will be presented, although ofttimes it has been 
necessary to investigate this region for a particular pig- 
ment. It is well to recognize the importance of such 
analyses in cases involving ultra-violet light. An ex- 
cellent example is zinc white which absorbs ultra-violet 
energy quite freely. 



354 COLOR AND ITS APPLICATIONS 

The investigation of the infra-red region requires 
a more elaborate apparatus although in many cases 
where total energy-absorption is of interest this can be 
obtained rather easily by means of the thermopile or 
bolometer. In fact, the ordinary radiometer or even 
the thermometer covered with a pigment yields data 
which have some uses in practice. Coblentz^ has 
published interesting data on the reflection-factors of 
various substances for infra-red and visible radiation 
of several wave-lengths. Among the substances which 
he studied were a number of pigments. The reflection- 
factors of white pigments for energy of wave-lengths 
4.4)Lt varied from about 0.1 to 0.4 and at S.S^t and 24/z 
were considerably lower. These data especially empha- 
size the localized nature of absorption-bands as, for 
example, cobalt oxide is a better reflector of long-wave 
energy than zinc oxide, yet for visible rays it possesses 
an extremely lower reflection-factor than zinc oxide. 
Lead oxide is a much more efficient reflector of long-wave 
energy than zinc oxide, magnesium carbonate and other 
white pigments. The importance of the infra-red analyses 
is apparent in many practical activities. Coblentz has 
pointed out that a pigment which has a low reflection- 
factor for energy of wave-lengths in the region of 8/x 
to 9/x is a better house paint in hot climes because it 
re-radiates maximally in this region where the maximum 
radiation from bodies of temperatures from 20° to 25° C. 
is found. If the paint has a high reflection-factor for 
visible rays it thus minimizes the heating effect of the 
incident energy. Such a combination is quite desirable 
in minimizing the heating effect of solar rays. This is 
merely one example of a vast number of interesting 
problems which could be met with more intelligence if 
spectral analyses were available. 

91. Some Optical Properties of Pigments. — In con- 



CERTAIN PHYSICAL ASPECTS AND DATA 355 

sidering the optical properties of a painted surface it is 
necessary to distinguish between a pigment and a paint. 
The former and its vehicle constitute the paint and the 
optical properties of a painted surface depend not only 
upon those of the pigment but also upon the vehicle and 
the surface covered. H. E. Merwin^ has made interest- 
ing studies of these properties. 

Most pigments, with the exception of lakes, consist 
of minute crystals and the color commonly varies with 
the direction of the passage of light through a crystal. 
Therefore, the shape of these crystals influences the 
value of a pigment. The transmission- (and absorption-) 
bands of pigment crystals are rather wide and shallow 
and small grains are more transparent than large ones 
of the same material. For this reason, a pigment con- 
sisting of small grains is generally brighter than one of 
large grains. Small grains may be considered to have 
diameters of the order of magnitude of 1/x and large 
ones of the order of lO/x. Usually the diameter of grains 
of colored pigments lies between 0.5^ and 10/x. 

The coloring power of a pigment generally increases 
as the size of the grain decreases but there is no definite 
relation covering different substances. For a given 
amount of pigment it is obvious that the total amount 
of surface exposed to intercept light increases inversely 
as the square of the diameter of the grains, although 
the ability of a grain to alter transmitted light in any 
direction increases more slowly than the diameter. 

Merwin considered four classes of colored pigments 
with regard to their adaptability to the making of tints 
and shades. They are as follows: 

a. Colored grains are chiefly of such size that if 
closely packed in a single layer they would transmit 
(or diffuse and transmit) a clear tint (say roughly 40 
to 60 per cent, white). From 5 to 20 such layers would 



356 COLOR AND ITS APPLICATIONS 

produce a full color. Either clear tints or pure shades 
can be made from such a pigment. Examples: chrome 
orange, chrome yellow, verdigris, ultramarine blue. 

b. Grains are so transparent that white light after 
traversing many layers of grains still contains a good 
deal (20 per cent, or more) white. Such a pigment can 
be used in making clear tints but not pure shades. Ex- 
amples: barium yellow, basic copper carbonate, stron- 
tium yellow. 

c. A single layer of grains absorbs several per cent, 
of the characteristic hue, and other hues almost com- 
pletely. Pure shades and dull tints may be made from 
such a pigment. Examples : vermilion, scarlet chromate, 
Harrison red, chrome green. 

d. Single grains absorb several per cent, of the 
characteristic hue and even several layers of grains 
do not absorb other hues completely. When darkened 
by a black pigment dull shades result, and when lightened 
by a white pigment dull tints are formed. Examples: 
Naples yellow, some Dutch pinks and yellow ochres. 

In the last two classes diffusing power determines 
to some extent and absorbing power to a greater extent, 
what range of pure shades can be obtained. 

Vehicles when dried have refractive-indices in the 
neighborhood of 1.5 and this indicates that the amoimt 
of light regalarly reflected from a smooth surface of a 
vehicle is about four per cent. A substance to be most 
effective as a pigment should have a high refractive- 
index for the hue it most freely transmits. The re- 
fractive-index varies considerably in the neighborhood 
of an absorption-band, being greater on the long-wave 
side than on the short-wave side. This is a reason for 
the greater refractive-indices usually exhibited by yellow, 
orange, and red pigments than by blue and violet. Of 
course, the refractive-index of a lake is largely deter- 



CERTAIN PHYSICAL ASPECTS AND DATA 357 

mined by the base and is usually comparatively low. 
If the refractive-index of a pigment closely matches 
that of the vehicle, the former will diffuse very little 
light. Such a pigment would ordinarily be mixed with 
one of higher refractive-index which will diffuse the light. 

Obviously, a black pigment to appear black in a 
dried vehicle should have the same refractive-index 
as that of the vehicle and it must absorb all the light 
incident upon it. In the dry state surrounded by air the 
pigment particles will reflect some light regardless of 
their absorbing power. Even when the refractive- 
indices of pigment and vehicle are equal, there is re- 
flected directly from the surface of the paint about 
4 per cent, of the incident light. To overcome this, 
light-traps such as possessed by a velvet may be pro- 
vided. Ivory black is an excellent black because its 
refractive-index is nearly the same as that of oil or 
varnish. 

From the foregoing consideration it is obvious that 
a high refractive-index is essential to a white pigment. 
The grains should be fine and there should be no selec- 
tive scattering of light of various wave-lengths. The 
burning vapor of metallic zinc produces very fine grains 
of zinc oxide. These are less than 1/x in diameter. 
Most of the zinc oxides contain enough fine grains less 
than Iju in diameter to give a bluish tint to paints by virtue 
of the selective scattering of light of the shorter wave- 
lengths. 

92. Some Applications of Spectral Analyses of Pig- 
ments. — The chief use of data derived from such 
spectral analyses is that of establishing the spectral 
character of the pigment. The general value of such data 
needs no defense, for it is the actual foundation of the 
pigment as a coloring material. Its purity is thus estab- 
lished; its influence in color-mixture may be predicted; 



358 COLOR AND ITS APPLICATIONS 

the purity or desirability of a color resulting from various 
mixtures of pigments whose spectral analyses are avail- 
able may be predetermined; and in many ways such 
data are useful. It is quite beyond the scope of a single 
chapter to discuss all the physical uses of such data, 
besides it is the intention to confine the discussion 
chiefly to aspects which are likely to be less commonly 
appreciated. For the latter purposes other data such 
as the spectral energy-distribution in illuminants and 
the visibility of radiation of various wave-lengths are 
necessary, therefore Table XXIII is presented. The 
relative energy-values at various wave-lengths are given 
for four illuminants which represent nearly the extremes 
commonly encountered from the viewpoint of color. 
In the last column are presented the visibility data^ 
standardized in the 1918 report of the Nomenclature 
and Standards Committee of the Illuminating Engineer- 
ing Society. There is no exact agreement as yet among 
investigators regarding the visibility of radiation of 
different wave-lengths, however, the data are sufficiently 
well established for the present purpose. On multi- 
plying each ordinate of a spectral energy distribution 
curve of an illuminant, pigment, dye, etc., by the cor- 
responding value of visibility, the resultant data yield 
the spectral Imninosity-distribution of the illuminant, 
pigment, dye, etc. Thus, from the spectral energy and 
visibility data the relative spectral luminosity-values can 
be determined. On integrating the areas of the spectral 
luminosity curves, the relative total liuninosity-values 
of colored media and of illuminants can be obtained and 
by dividing the area of one of the former by the area of 
one of the latter, the reflection-factor of the particular 
colored medium is obtained for the particular illuminant. 
Thus by computation, the reflection-factors of colored 
media can be obtained without any of the difficulties and 



CERTAIN PHYSICAL ASPECTS AND DATA 



359 









TABLE 


xxm 






Spectral Energy-Distribution in Common Illimunants and the 

of Radiation 


Visibmty 








Tungsten 
(vacuum) 


Tungsten 
(gas-filled) 


Visibility of radiation* 


Wave- 


Blue sky 


Noon sun 


Incandescent 
Lamp 


Incandescent 
Lamp 






length 




AhcnIiifA 








7.9 lumens 


22 lumens 


Relative to 


(Lumens per 
watt) 








^att 


watt 


that at 656mm 


0.40/x 


170 


67 


9 


15 


0.0004 


0.0000006 


.41 


177 


72 


9.5 


16.6 


.0012 


.0000018 


.42 


181 


75 


10.5 


19 


.0040 


.0000060 


.43 


185 


79 


12 


23 


.0116 


.000017 


.44 


186 


83 


16 


26.6 


.023 


.000034 


.45 


187 


84.3 


16.7 


30 


038 


0.000057 


.46 


185 


88 


20 


33.7 


.060 


.000090 


.47 


180 


91 


23.6 


38 


.091 


.000136 


.48 


173 


92 


27 


42.6 


.139 


.000208 


.49 


162 


92.5 


32.7 


47 


.208 


.000312 


.60 


157 


95 


37.6 


62 


323 


00048 


.61 


146 


96 


42.6 


56.6 


.484 


.00073 


.52 


140 


97 


49 


62 


.670 


.00100 


.53 


132 


98 


54.9 


67 


.836 


.00126 


.64 


127 


99 


62.1 


72.6 


.942 


.00142 


.66 


120 


99 


68.6 


78 


0.993 


0.00149 


66 


115 


100 


76 


83 


.996 


.00149 


.57 


108 


100 


83.4 


88 


.952 


.00143 


.68 


104 


101 . 


91 


94 


.870 


.00130 


.69 


100 


100 


100 


100 


.757 


.00114 


.60 


97 


100 


108 


105 


631 


0.00095 


.61 


93 


100 


117 


111 


.503 


.00075 


.62 


90 


99 


126 


116 


.380 


.00057 


.63 


87 


98.6 


136 


121.5 


.262 


.00039 


.64 


85 


98 


146 


126 


.170 


.00025 


.65 


82 


97.1 


157 


131 


0.103 


0.000154 


.66 


80 ' 


96 


167 


135 


.059 


.000089 


.67 


77 


95 5 


179 


140 


.030 


.000046 


.68 


76 


94 


189 


143 


.016 


.000024 


.69 


72.5 


93.5 


202 


147.5 


.0081 


.0000122 


.70 


71 


91.7 


212 


151 


0.0041 


0.0000061 


.71 


69.6 


90 


223 


153.6 


.0021 


.0000031 


.72 


68 


88 


236 


156 


.0010 


.0000016 



* Standardized in 1918 Report of Committee on Nomenclature and Standards 
of I.E.S. 



uncertainties of color-photometry, for these have been 
involved in the determination of the visibility data. 
Such computations are found to yield results quite in 



360 



COLOR AND ITS APPLICATIONS 



agreement with those obtained by direct measurement 
of reflection- (or transmission-) factor. In fact, this 
method appeals very strongly to the author, especially 
because the spectral analyses should be available for 
many other reasons so that reflection- and transmission- 
factors would be by-products. 

The spectral luminosity-distributions of the visible 
radiation reflected from pigments whose spectral re- 




a44M H6 



J4 .^6 ^ j60 

Fig. 133. — Pigments. 



flection-factor distributions are shown in Figs. 131 and 
132 and in Table XXII are presented in Figs. 133 and 134. 
These may also be considered as the spectral reflected- 
energy distributions for an imaginary illtmiinant of 
uniform spectral energy-distribution. Incidentally, the 
light from the noonday sun approaches this ideal fairly 
closely as seen by Table XXIII for in this table the 
energy-values of this ideal illuminant would be 100 for 
all wave-lengths in order to be directly comparable 
with the other illuminants. 



CERTAIN PHYSICAL ASPECTS AND DATA 



361 



93. Reflection-factor of Pigments. — In order to 
cover the general case more accurately much of the fore- 
going discussion will be expressed mathematically but, 
for the sake of clearness, reference will be made to 
these various curves in Fig. 135 for a specific case. 
/ = Spectral energy-distribution of 
an illuminant (tungsten fila- 
ment at 7.9 lumens per watt). 



JO 



J- ff/iH^ J/^A'AC-* 

//- COB^C T BLue 
































































^ 


-^ 
















1 




/,. 


^L.^ 


\ 














1 


1 


/ 

/ 




\ 

V 














ii 
^ 




/ 




>. 


\ \ 
\\ 












^^ / 




/ 




N 


^^ 


\ 












^ 







A 


\ 


\ 












j:^ 


:*-*^ 


--■^** 




-'^ 


s 


^ 


^^?*. 






Q.44M 46 46 JO ^ 


^ ^ 




'£ Lf/V 


3 j6 
fT/f 


^ 


^^ 6 


4 1 


T^i 


«s y 


p J^ 



Fig. 134. — Pigments. 

J\ = Energy-value of the illuminant at 
any wave-length, X. 

V = Visibility curve. 

Kx = Visibility-value for energy of 
wave-length, X. 

Li = Spectral luminosity-distribution 
of illuminant /. 

P = Spectral reflection-factor distri- 
bution of a pigment (light 
chrome yellow). 



362 



COLOR AND ITS APPLICATIONS 



R\ = Reflection-factor of the pigment 
for energy of wave-length, X. 

Lp = Spectral luminosity-distribution 
of radiant energy reflected 
by the pigment. 



For X = 0.52/z, ad 
and ab = RxKxJx. 



Jxy af = Kxy ae = Rxy ac = KxJ 



x> 



/^ 



1^0 



1^ 

















/ 














/ 




> 










i/ 






1 








/ 








is 
1 






/ .^ 


K" 





p_ 


— 








u 


-^ 


Sjx^ 






< 

.« 




/ 


I'i 










i;^'^ 




4> 


1 
b 






V 





Q^OjU ^4 



sW 



^Z J6 j60 



M 



.6d 



r 



Fig. 136. — Analysis of a pigment. 

KxJxdX = Area enclosed by Lj, which is propor- 
Xi tional to the total luminous flux E, received by the 
surface between limits Xi and X2, hence is equal to CE 
where C is a constant of proportionality. If the total is 
desired, the limits, Xi and X2, are respectively the limits 
of the visible spectnmi which for most practical cases 
may be taken as OA/jl and 0.7^. 



/ 



RxKxJ\d\ = Area enclosed by ^p, which is pro- 
Xx portional to the total luminous flux, E\ reflected 
by the surface (pigment P) and is equal to CE'. 






CRETAIN PHYSICAL ASPECTS AND DATA 363 

If energy is of interest instead of lixminosity, Kx is 
eliminated and the limiting wave-lengths Xi and X2 are 
given the desired values. 






E 



f 



^ = -^ = R= the reflection-factor of the 

' KxJxdX pigment P for the illuminant /, 

and Xi and X2 are respectively the 

wave-lengths at the limits of the 

visible spectrum. These limits could be expressed as 

and 00 without changing the result because beyond the 

visible spectrum Kx is zero. 

Many useful data can be obtained by such computa- 
tions when the spectral energy-distributions of pigments 
and of illuminants are available. These computations 
can be made for a sufficient number of wave-lengths 
throughout the spectrum and the relative values of the 
integrals can be obtained by means of a planimeter 
from the plotted curves or more readily by stmunating 
the computed values. 

Similar computations have been made for the group 
of pigments already introduced for four illuminants 
including the ideal having a uniform spectral energy- 
distribution. These values are presented in Table XXIV 
and Fig. 136. The values are given to the third decimal 
place not with the belief that the absolute values are 
determined with such accuracy but to show the differ- 
ences as accurately as possible obtained by this method 
of computation. The relative values are perhaps accurate 
to the third place. It is seen that the reflection-factor 
for a given pigment i^ not constant (see # 42) but 
varies with the illuminant. This is a point not gener- 
ally appreciated and inasmuch as this difference exists 
the suggestion is made that, for general purposes, 
reflection-factors be given for an illuminant of uni- 



364 



COLOR AND ITS APPHCATIONS 






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CERTAIN PHYSICAL ASPECTS AND DATA 



365 



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366 



COLOR AND ITS APPLICATIONS 






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(sat.) 

Cobalt chromate 



CERTAIN PHYSICAL ASPECTS AND DATA 



367 



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368 



COLOR AND ITS APPLICATIONS 



form spectral energy-distribution. In cases of direct 
measurement of these factors the clear noon-day sim 
sufficiently approaches the ideal as will be shown shortly. 
In cases where other illuminants are used these should 
be specified. 

The measurement of reflection-factor directly is by 
no means standardized and in the case of colored pig- 




Fig. 136. — Pigments. 

ments this measurement is attended with many diffi- 
culties such as the distribution of Imninous flux upon 
the surface, its angular position with respect to the 
photometer, color-photometry, etc. A discussion of this 
has been presented elsewhere.^ 

In Table XXIV the relative reflection-factors of each 
pigment by itself for the four illuminants are presented, 
that for the uniform energy-spectrum being taken as 
unity. This gives a better idea of the magnitude of the 
variation of the reflection-factor with the spectral char- 
acter of the illuminant. These values are plotted in 
Fig. 137 and as would be expected, the red and yellow 



CERTAIN PHYSICAL ASPECTS AND DATA 



369 



pigments show relatively greater reflection-factors for 
tungsten light than for blue sky-light with the values 
for sunlight (circles) lying between. It is interesting to 
note the proximity of the circles to imity which repre- 
sents the relative value of reflection-factor in. each 
case for the ideal illuminant having an uniform spectral 
energy-distribution. 

The effect of the illuminant upon the appearance of 



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Fig. 137. — Pigments. 

the color is shown in Fig. 138 using, for example, ultra- 
marine blue whose spectral energy-distribution is shown. 
The spectral luminosity-distributions of this pigment 
for the different illuminants have been computed for 
equal total amount of reflected light (enclosed areas 
equal). Thus an idea of the appearances of the color 
can be formed or conversely the reason for these differ- 
ent appearances under the three illuminants is manifest. 
Incidentally, it is seen that the pigment is of a purer 
color under blue skylight than tmder either of the other 
illuminants. The wave-length of maximum luminosity 
is 0.495m and 0.54^ respectively for the skylight and 
tungsten light. This wave-length of maximum lumi- 



370 



COLOR AND ITS APPLICATIONS 



nosity is not necessarily the dominant hue of the color 
as analyzed by the eye or by the monochromatic color- 
imeter although these are often nearly coincident. 

94. Spectral Analyses of Dye-solutions. — The mix- 
ture of dyes is governed by the same subtractive 
principles of color-mixture as the mixture of pigments 
although the greater number of dyes and the more 



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i& ^ .^ j60 

Fig. 138. — Ultramarine. 



exacting or delicate applications of dyes in industries, in 
the making of accurate filters, etc., make their spectral 
analyses of perhaps more importance than in the case 
of pigments. Certainly, a knowledge of the spectral 
characteristics of dyes, as in the case of pigments, makes 
for an ease and certainty in making and in visualizing 
mixtures which cannot be enjoyed without such data. 
It is beyond the scope of this section to present a com- 
plete discussion of the usefulness of spectral analyses 



CERTAIN PHYSICAL ASPECTS AND DATA 371 

of dyes or to present the spectral analyses of all the dyes 
available; however, a few representative analyses of 
dyes most common and perhaps most reproducible 
should be of value. These are presented in the following 
tables roughly classified as to color. The highest ac- 
curacy is not claimed for these data because it does not 
appear worth the effort necessary because there is no 
indication that these dyes are in general constant in 
spectral characteristics as obtained from time to time 
in the market. For the same reason it has not been 
considered necessary to give values of concentration. 
From the data presented in the tables it is possible 
to obtain an idea of the spectral characteristic of a given 
dye-solution for any depth of the particular concen- 
tration employed and also for any relative value of con- 
centration. In other words, from the data in the tables 
and the discussion which follows it is possible to be 
guided in the selection of dyes for many purposes. 
For the study of a dye-solution throughout an entire 
range of depth and concentration by the method de- 
scribed later, the spectral analysis should be obtained 
as accurately as possible. In all cases where not 
indicated otherwise, the solvent was distilled water. 
The dyes were obtained from various well-known com- 
mercial sources. Among the solutions will be found a 
few solutions of metallic salts which are incorporated 
for their usefulness as filters. All data have been cor- 
rected for surface reflections and for the absorption of 
the glass cell by the method of substitution. 

In Table XXV are presented the spectral analyses 
of a number of dye-solutions commonly classed as red 
although many are purple. The sharpness of the absorp- 
tion- or transmission-bands is readily visualized from 
the data although it is of advantage to plot the data in 
many cases. There are some excellently sharp bands 



372 COLOR AND ITS APPLICATIONS 

shown, for example, that of eosine of moderate con- 
centration. In some cases spectral analyses for two con- 
centrations have been presented. 

In Table XXVI spectral analyses of a number of 
yellows are presented. It is noteworthy that there is no 
known dye which transmits only a narrow region near 
spectral yellow. The value of sharp absorption-bands 
is seen when a fairly monochromatic filter is desired. 
For instance a yellowish green dye with a sharp cut-off 
on the long-wave side combined with a greenish yellow 
dye with a sharp cut-off on the short-wave side will 
yield a fairly monochromatic green filter. Some of the 
dyes fluoresce which from the point of view of color alone 
is of considerable interest. Fluorescein and uranine 
are among the many which fluoresce strikingly. It is 
interesting to study these by projecting a spectrum upon 
their upper liquid surface and by viewing the result 
both from above and from the side. The spectral analyses 
of potassitmi bichromate and cobalt chromate are in- 
cluded. 

Among the greens in Table XXVII are a number of 
dichroics. In fact, a very common characteristic of 
green dyes is the exhibition of dichromatism. This can 
readily be ascertained by noting the energy-spectrum 
or spectral transmission characteristic of one of these 
dyes. If the transmission-factor for red, say 0.7^t, i§ 
in any one case greater than that for any wave-length 
in the other regions of the spectrum (in the green for 
so-called green dyes) the solution at great depths or 
concentrations will appear red and therefore will be 
dichroic. Naphthol green is an excellent yellowish green 
dye. Among the greens presented, malachite, saurgriin, 
methylengriin, and neptune green exhibit dichromatism. 

One of the most annoying features of dyes is the 
extreme rarity of pure blue dyes. Nearly all blue dyes, 



CERTAIN PHYSICAL ASPECTS AND DATA 373 

Table XXVIII, transmit the extreme red rays quite 
freely and the scarcity of blue-green dyes which are not 
dichroic makes it diffictdt often to find a combination 
which transmits only the violet rays. In extremely 
high concentrations or great depths some blue dyes 
effectually absorb most of the extreme red rays. 

In Table XXIX are presented a number of spectral 
analyses grouped under the common name of purple 
for the purpose of classification. An interesting case 
is that of ethyl violet in gelatine both wet and dry. After 
the dyed gelatine, which was flowed on clear glass, had 
set, and while still wet the spectral analysis was made. 
The sample was then allowed to dry and another spectral 
analysis was made. On plotting these data a decided 
difference in the spectral transmission curves is seen 
as indicated by the numerical data. The wet specimen 
is decidedly more reddish than when dry and an actual 
shift in the absorption-band takes place on drying. 
Although not definitely established this may be ex- 
plained as due to a difference in the refractive-index 
of the solvent in the two cases. The data are corrected 
for reflections from the gelatine and glass surfaces. 

In Table XXX are presented spectral analyses of 
dyed gelatine filters before and after fading by exposure 
to solar radiation. Such data are of special interest in 
many cases and it appears of interest to make a thorough 
study of the fading of dyes with the aid of spectral analy- 
ses. Certainly no great amount of information is avail- 
able regarding the relation of the spectral character 
of radiation to the spectral deterioration of dyes or the 
relation of either of these to the chemical composition. 
Incidentally, the testing of dyes under illuminants 
containing ultra-violet rays of extremely short wave- 
lengths which are practically absent in solar radiation 
at the earth's surface or in artificial illuminants as com- 



374 



COLOR AND ITS APPLICATIONS 



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CERTAIN PHYSICAL ASPECTS AND DATA 



375 



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376 



COLOR AND ITS APPLICATIONS 



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O H M ^ S p< 



CERTAIN PHYSICAL ASPECTS AND DATA 377 

monly encountered is open to criticism. Spectral analy- 
sis has not been sufficiently utilized in permanency tests 
to warrant all the conclusions which have been drawn 
in this matter although some excellent work has been 
done.^ Mott has shown that the results with the * snow- 
white ' flame arc in dye-fading are practically the same 
as those obtained in daylight. He states that the white 
flame arc at 25 amperes affords light at a distance of two 
feet more intense than simimer sunlight. By focussing 
the image of a quartz mercury arc by means of a quartz 
lens, an intense illumination rich in ultra-violet rays 
may be obtained. The large incandescent lamps may 
also be used with success. 

95. Applications of Spectral Analyses of Dyes. — The 
uses for spectral analyses of dyes are manifold, as in 
the case of any class of colored media. In general, they 
provide a physical basis for systematic color-mixture be- 
sides providing the necessary information for choosing 
dyes for many purposes. In many aspects of color- 
technology only the integral or subjective color is finally 
of interest but the author cannot refrain from empha- 
sizing that even in such cases an intimate knowledge 
of colored media and theii; mixture cannot be attained 
without spectral analyses and that the combination of 
dyes becomes systematic with such data available. 

With spectrophotometric apparatus well maintained, 
a complete spectral analysis can be made in about an 
hour although there is much room for improvement in 
such apparatus which will result in the saving of time. 
However, this is not a serious matter because for a 
given coloring material only one anlaysis need be made, 
as will be shown later, to provide information for all 
degrees of concentration or depth of solution. The 
author has available hundreds of spectral analyses which, 
after once obtained, are a perpetual source of information. 



378 COLOR AND ITS APPLICATIONS 

96. Laws Pertaining to Colored Solutions. — In order 
to simplify the study of coloring media, especially dyes 
and colored glasses, several simplifications have been 
made. These are based on theory and have been con- 
firmed by experiment on a few typical specimens. In 
order to develop this procedure it is necessary to revert 
to some of the established laws. Lambert first stated 
that all layers of equal thickness of a transparent medium 
absorb equal fractions of the radiant energy which enters 
them. This is true for homogeneous or monochromatic 
radiation, but cannot be applied to the total absorption 
of radiant energy of many wave-lengths or of extended 
spectral character. 

It follows from Lambert's law that if the thickness 
of the absorbing medium increases in arithmetical 
progression the radiation transmitted should decrease 
in geometrical progression. 

Let J be the intensity of radiation of a given wave- 
length entering a layer dl^ then— 

dl ^ 

On integrating this we obtain, 

J=Joe-^^ 

where Jo is the original intensity, J the intensity after 
traversing a thickness c/, and fe is a constant depending 
upon the substance and upon the wave-length of the 
radiant energy. Various terms have been applied to 
this factor such as absorption-index. In logarithmic 
form this equation is expressed as. 

Log — = log Tx = - k\d log e = - exd 

Jo 

where Tx is the transmission-factor for energy of wave- 
length X, and the subscripts, X, indicate the factors which 



CERTAIN PHYSICAL ASPECTS AND DATA 379 

vary with the wave-length. Beer deduced the law that 
the absorption is the same function of the concentration 
of a dispersing absorbing substance as of the thickness 
of a single substance which may be expressed thus: 

J = JoAx"^ or T\ = Ax^ or log Tx = cd log A\ 

where c is the concentration, A is the transmission- 
coefficient or transmissivity and the other symbols repre- 
sent the same factors as in the foregoing equations. 
The validity of Beer's law has been questioned by some 
and it appears that there is some doubt as to its validity 
in such cases as colloidal solutions. This law appears 
to hold when the absorbing power of a molecule is un- 
influenced by the proximity of other molecules. Ob- 
viously, if any change takes place in the condition of the 
dispersed substance on altering the concentration the 
law will not hold. Incidentally, there is work to be done 
on the validity of this law in the cases of * colloidal' 
glasses. Lambert's law appears to be firmly established. 

In so far as the foregoing laws are valid (and it ap- 
pears that this is true for all practical purposes such 
as described in this chapter) for a given solution long Tx 
is proportional to cf, and for a given depth, or containing 
cell, log T\ is proportional to c. By the use of coordinate 
paper having a logarithmic scale along one axis and a 
uniform scale along the other, a great deal of interesting 
data can be obtained from one spectral analysis. 

By means of the foregoing mathematical relations 
the spectral analyses of colored solutions (and colored 
glasses) of any thickness and concentration can be 
obtained from two determinations of spectral character 
which may be reduced to a single determination. Such 
a method has been found exceedingly practicable in 
preliminary reconnoitering in search of combinations 
of dyes for filters, in the development of colored glasses, 



380 COLOR AND ITS APPLICATIONS 

and in the study of many problems arising in color- 
technology. 

Some examples will suffice to illustrate the uses of 
this scheme in practice. Assume a solution of methyl- 
engriin of either known or unknown concentration. A 
cell of a known thickness is fiUed with the solution 
and a spectral analysis is made. For such a purpose a 
fairly low concentration or small depth is chosen so that 
radiations of all wave-lengths which are of interest are 
appreciably transmitted. On logarithmic paper as pre- 
viously described, a plot is made as shown in Fig. 139, 
the transmission-factors from the spectral analysis being 
plotted on the logarithmic scale vertically above the 
arbitrarily selected point on the abscissa axis in this 
case taken as unity. The abscissae scale may represent 
either concentration or depth and may be either a relative 
or an absolute scale. Straight lines are drawn through 
the points to a common point on the ordinate axis 
representing complete transparency or unity on this 
logarithmic scale. This is the common point if correc- 
tions have been made for surface reflections in the cell 
or from the glass surfaces in the case of a colored glass. 
If these corrections have not been made, the conmion 
point usually will be near 0.92 on the * transmission axis' 
if two surface reflections must be accoimted for. Each 
straight line represents the relation of log Tx and depth 
or concentration for a certain wave-length. By extend- 
ing these lines the spectral characteristic of any depth 
or concentration may be read from the corresponding 
vertical line. If the original spectral analysis has been 
made with care such a simple plot jdelds a vast amount 
of data. 

97. Dfc/iromafem. — Methyl engriin has been 
chosen in Fig. 139 because it also illustrates the inter- 
esting case of dichromatism so commonly exhibited by 



CERTAIN PHYSICAL ASPECTS AND DATA 



381 



dyes. It is seen that the slope of the line for 0.72jjl is 
less than any of the others. This is proof that the dye 
is a dichroic. Some lines are very steep which indicates 
a large value of the extinction coeflSicient A for radiation 
of these wave-lengths. From the plot it is seen that this 
dye, in solutions of high concentration or of great depth, 
will not be green but will be red. 

Another interesting plot, of a similar nature but in- 



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Fig. 139. — Methylengriin solution. 

eluding relative luminosity-values instead of transmis- 
sion-factors is shown in Fig. 140 for rosazeine. The 
spectral transmission-factors for the spectral analysis 
used were multiplied by the visibility of radiation in 
each case and plotted vertically above the point desig- 
nated by unity on the concentration or depth scale. 
Instead of drawing straight lines representing various 
wave-lengths to a common point on the ordinate axis, 
each line is drawn to a point of this axis corresponding 
to the relative visibility of radiation of the particular 



382 



COLOR AND ITS APPLICATIONS 



wave-length. The ordinate axis is now a logarithmic 
scale of relative luminosity. By extending these straight 
lines a graphical picture of spectral limiinosity of the 
dye-solution is obtained for any concentration of depth. 
It is seen that this solution in great depth or high 
concentration becomes deep red because the slopes 
of the lines become less with increasing wave-length 



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after the absorption-band of a weak solution or small 
depth is passed. Incidentally, it will be noted that the 
slope of line 0.44^ is less than that of 0.58/x which shows 
that in low concentrations or in relatively small depths 
of a higher concentration the solution is purple, that is, 
it has an absorption-band somewhere between 0.44/x 
and 0.60/x. Only a few wave-lengths have been used 
for the sake of clearness. 

98. Complete Representation of the Graphical 
Method. — In reality the schemes illustrated in Figs. 139 



CERTAIN PHYSICAL ASPECTS AND DATA 



383 



and 140 are only completely illustrated by means of a 
solid of which, for example, Fig. 139 represents a pro- 
jection upon the face of the solid boxmded by the logarith- 
mic * transmission' scale and the concentration or depth 
scale. A model of this tri-dimensional diagram can be 



/i<4i^e leA/arfi 




40m -^Q ~^<^ •<i4 



Fig. 141. — Graphical representation of laws of spectral transmission. 

easily made and should be instructive. An attempt is 
made in Fig. 141 to illustrate the relations between 
transmission-factor, wave4ength and concentration or 
thickness. For this purpose the spectral analysis of a 
thin piece of gold red glass was chosen. Many of the 
cross-section lines have been omitted for the sake of 
clearness. The scales are designated and the thickness 



384 COLOR AND ITS APPLICATIONS 

of the specimen of gold ruby glass is assumed to be 
2 units on the relative thickness scale. In plane 2 repre- 
sented by the dash-dot vertical rectangle, the spectral 
transmission is shown in the dash-dot curve 02^202. 
For the limiting case of zero thickness, this curve be- 
comes a straight line, T = 1, which is the top edge of the 
foremost rectangle, plane 0. Several points of the 
'master' curve in plane 2, were taken for the purpose 
of illustrating the determination of the spectral char- 
acteristic of the glass at another thickness. In this 
example, thickness 5 units is taken and its spectral trans- 
mission is shown by the dotted curve in plane 5, the 
farthest vertical rectangle. This curve is obtained by 
drawing straight lines in the * wave-length' planes from 
the wave-lengths on the upper front scale through the 
points on the * master' curve in plane 2 of corresponding 
wave-lengths. Thus where a given straight line inter- 
sects the various thickness planes, the transmission- 
factors for that wave-length are found. For example, 
&2 is a point on the * master' curve in plane 2 and its 
value as read from the transmission-scale is the trans- 
mission-factor of this specimen of thickness, 2 units, 
for radiation of wave-length 0.52/1. A straight line 
drawn through this wave-length on T = 1 and through 
62 (always remaining in the particular wave-length 
plane) when prolonged intersects plane 5 at br, which 
is the transmission-factor for 0.52/>t for a specimen of 
the same glass of 5 units thickness. Other points, a^, 
C5, etc., are found in the same manner. 

These straight lines are the same as those shown in 
Fig. 148; in fact. Fig. 148 would be seen on viewing the 
solid. Fig. 141, from the righthand side. A model of 
this solid made of wires and painted to represent the 
spectral colors should be instructive. 

In Fig. 140 luminosity-values were treated in a 



CERTAIN PHYSICAL ASPECTS AND DATA 385 

manner similar to the transmission-values in Fig. 139. 
These also can be completely represented by a solid 
in a manner similar to that shown in Fig. 141 excepting 
that the vertical scale must represent logarithms of 
luminosity. In the limiting case of zero thickness the 
curve will not be the upper foremost horizontal line but 
will be the spectral luminosity-curve of radiation and 
will lie in the foremost vertical plane. On viewing such 
a solid in projection from the proper side, Fig. 149 will 
be seen if the same gold ruby specimen be taken as an 
example. It appears unnecessary to illustrate this 
possibility since the general procedure should be under- 
stood from the foregoing. In case the analysis is to be 
made for a particular illuminant the limiting curve in the 
foremost vertical plane will be the luminosity curve of 
the illuminant. 

One of the points which is emphasized in dealing 
with colored media in the foregoing manner is that the 
spectral transmission- and reflection-factors are never 
zero but are merely relatively low for some wave-lengths 
as compared with others. This is often forgotten when 
spectral analyses are made with instruments because 
when the luminosity falls below the threshold the trans- 
mission-factor is considered to be zero; however, 
the threshold depends upon the intensity of illumination 
or upon the brightness of the light-source. 

99. Spectral Analyses of Glasses. — In the develop- 
ment of colored glasses for the variety of practical appli- 
cations, the spectral analyses are extremely valuable 
and often essential. By means of such data these color- 
ing elements can be mixed computationally to obtain 
the desired spectral characteristic. From very meagre 
data on the chemical composition from one melt, fairly 
definite strides toward realization may be made in 
succeeding melts. Of course, there are chemical con- 



386 COLOR AND ITS APPLICATIONS 

siderations which sometimes alter the predictions based 
on computation; however, such a procedure forms a 
most definite working basis. In the combination of 
glasses for special filters, lighting effects, etc., the com- 
putational method often saves time and provides definite 
data. Sometimes only the subjective color is desired 
but even in these cases spectral analyses of elemental 
colorings provide the basis for manipulating the avail- 
able vitrifiable colored media in a manner analogous 
to the combination of pigments. 

In the manufacture of colored glass there is a limited 
number of coloring materials available and when the 
glass must be limited to one general composition, such 
as soda lime, for example, the colors which are possible 
of attainment are further limited. However, by combin- 
ing various coloring materials, the variety of colored 
glasses can be enormously extended to meet the require- 
ments of science and art. 

In this chapter the spectral analyses of a few funda- 
mental colored glasses will be presented and also the 
results of a few simple combinations. The record num- 
ber of the specimen is placed before the s3mibol of the 
coloring metal such as 37 Se. If different relative thick- 
nesses of the specimen are presented, a number is placed 
before the designation proper as 10(37 Se) indicates 
10 tmits of thickness (or of concentration); CS indicates 
lime soda gldss; PS, lead soda; BS, barium soda; P, 
lead, etc. 

100. i?ec?. — Selenium, copper, and gold are com- 
monly used for producing red glasses. In Fig. 142 are 
shown the spectral analyses of a number of selenium 
glasses. It is seen that some of these are yellow in 
appearance, varying from this to a deep red. The com- 
position of the mix is sometimes of considerable influence 
upon the final color. Specimen 14 Se shown in relative 



CERTAIN PHYSICAL ASPECTS AND DATA 



387 



thicknesses, 10, 20, and 34 was of unknown composition 
but the coloring element was selenium. This is a re- 
markable specimen. Cobalt blue glass (Fig. 146) trans- 
mits a deep red band, so a combination of dense cobalt 
blue and selenium glasses isolates a deep red band as 
seen in 6 Co + 14 Se, Fig. 142. 

By computations similar to those presented in the 



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case of pigments (substituting transmission-factors, Tx, 
for reflection-factors Ry) the efficiency of such a com- 
bination in transmitting only a deep red band can be 
compared with that of a very dense seleniimi, gold, or 
copper glass. Unfortunately, at the ends of the visible 
spectrum the visibility data are least accurate; however, 
such relative comparisons by computation are depend- 
able. Incidentally, Hyde, Cady, and Forsythe^ have 
determined the visibility at the extreme red end of the 
visible spectrum with great care and Hartmann^ at 
the blue end. 



388 



COLOR AND ITS APPLICATIONS 



In Fig. 143 is shown the spectral characteristic of a 
copper red glass, 4 Cu, and in Fig. 144, the spectral 
analyses of gold glasses are presented. Gold produces 
a beautiful pink in low concentrations (or in thin layers) 
and deep red in the higher ones (or in thicker layers). 
The absorption-band is seen to be near 0.53/i for the more 
transparent glasses and it is interesting to note glass 




■5S 



Fig. 143. — Copper, sulphur, uranium, and chromium glasses. 

35 Au, a lead gold, which shows a shift in the absorption- 
band to 0.50iU. This glass was reheated several times 
in bringing out the color which is decidedly more ruddy 
and it appears that there is a different state of division 
of the metallic particles perhaps as to size. As the 
concentration or thickness increases (glass 5 Au, which 
is shown for three thicknesses) the blue band gradually 
disappears; however, the transmission does not closely 
approach monochromatism. In Fig. 144 are also shown 
the results of combining cobalt and gold glasses of dif^ 
ferent thicknesses (or concentrations), with the resulting 
transmission confined to the deep red region. 



CERTAIN PHYSICAL ASPECTS AND DATA 



389 



101. Yellow. — Carbon, sulphur, uranium, and silver 
are among those elements which, when introduced into 
glass imder proper chemical conditions, produce yellow 
glasses of varying color. No single element isolates 
spectral yellow. In Fig. 145 are shown the spectral 
analyses of carbon yellow glasses, 15 C and 44 C, and of 
combinations of carbon yellow and light cobalt blue 



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Fig. 144. — Gold and cobalt glasses. 

glasses. It is known that X-rays, ultra-violet and visible 
rays will cause some clear glasses to become colored. 
In Fig. 145 is also shown the spectral characteristic of a 
glass X, which though originally clear was colored a 
muddy yellow throughout the mass by X-rays. It is 
interesting to observe the action of X-rays in discoloring 
glass, for it is easy at times to observe the progress of 
the coloring through the thickness of the glass. Pat- 
terns can be made by this process. In Fig. 143 are shown 
the spectral characteristics of uranium (11 U) and sul- 
phur (43 S and 45 S) glasses. The spectral transmissions 



390 



COLOR AND ITS APPLICATIONS 



of several uranium samples appear to be kinky in the 
blue-green region although the exact nature of the 
curves are not established. 

102. Green. — Iron imparts a green color to glass 
varying from a bluish to a yellowish green, depending 
upon the ingredients of the glass. The importance of 
manganese in glass is as a decolorizing agent, its color 



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in proper concentrations being roughly complementary 
to that of iron commonly present in sand. Chromium 
imparts a yellowish green color to glass as seen in glass 
53 Cr, Fig. 143. This glass has a maximum transmission 
at about O.SGm and by the addition of copper blue-green 
(glasses 2 Cu and 8 Cu, Fig. 143) this maximum can 
be shifted toward the shorter wave-lengths depending 
upon the proportions of the coloring elements. Glass 
21 Cu, called signal blue-green, is evidently a copper 
glass. Glass 36 Cr is a dense chromium green. In 
order to compare the actual colors under a given il- 



CERTAIN PHYSICAL ASPECTS AND DATA 



391 



lixminant it is well to reduce these curves to luminosity 
values. If monochromatism is desired it is often ad- 
visable to combine two glasses which transmit a narrow 
region in common. 

103. Blue.— Cobalt is the most common element 
used to impart a blue co or to glass. Its greatest dis- 
advantage (although sometimes an advantage) is its 



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Fig. 146. — Cobalt glasses. 

transmission of a deep red band as shown in 6 Co and 
7 Co, Fig, 146. This red transmission can be utilized in 
isolating the deep red as shown by combining cobalt and 
selenium or other red glasses, for example, 1 Co + 14 Se. 
An excellent blue glass can be made by combining 
cobalt with copper blue-green, for the latter effectively 
absorbs the deep red. The spectral characteristic of 
such a combination is shown in 9 Cu + 6 Co, Fig. 146. 
104. Purple — Nickel produces a purple color in 
glass and also manganese but the latter is not an efficient 
purple because its absorption-band is not sharp. Its 



392 



COLOR AND ITS APPLICATIONS 



chief use is to neutralize the green tint due to the pres- 
ence of iron in the ingredients of glass mixes. The 
spectral characteristic of a glass containing iron is shown 
in 41 Fe, Fig. 147. It is seen that a manganese glass of 
proper density is approximately complementary in color 
to the iron glass. Although the manganese neutralizes 
the iron in color, the transmission-factor of the resultant 




OAXm 44 A6 



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Fig. 147. — Manganese and iron glasses. 



glass may be seriously reduced. Manganese, though 
a useful element in glass manufacture, cannot be con- 
sidered important as a coloring element from the view- 
point of colored glasses in general. In X, Fig. 147, is 
shown the spectral characteristic of an originally clear 
glass which has been colored a deep purplish hue by 
exposure to X-rays. Undoubtedly this coloring is due 
to an effect upon the manganese present in the clear 
glass. This effect is commonly observed in lamp globes 
and window glass exposed to strong sunlight. In the 
former cases it is a very serious defect of glass manu- 



CERTAIN PHYSICAL ASPECTS AND DATA 



393 



facture because the author ^^ has observed such globes 
whose transmission has been reduced as much as 50 per 
cent, after long exposure to intense solar radiation or 
to that emitted by an arc lamp. It would be far better 
in such cases as street-lighting glassware to eliminate 
the manganese and to endure the unneutralized greenish 
hue of the iron which is unavoidably present. 



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105. Use of Spectral Analyses of Glasses. — The ap- 
plications for spectral analyses of colored glasses have 
been fairly well covered in the discussions of pigments 
and dyes, for the same general procedures can be applied 
to colored glasses. The concentration is not so definite 
as in the case of dyes because, owing to the high temper- 
ature at which glass melts and to chemical action, the 
concentration of coloring material in the final glass can- 
not always be predicted from the amount of coloring 
metal added to the mix. Some of the metals such as 
cobalt and copper, tmder standardized conditions of 



394 



COLOR AND ITS APPLICATIONS 



melting, appear to produce concentrations of coloring 
material proportional to the amounts of the oxides added 
to the mix but in some cases there is doubt as to this 
proportionality. There is need for systematic study in 



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Fig. 149. — Gold ruby glass. 

this direction. In the case of the red glasses, for example, 
gold ruby, which in ordinary manufacture assumes its 
red color on reheating, the manipulation has consider- 
able effect upon the density of the color. After a colored 
glass has been obtained it is possible to procure from a 
single spectral analysis the integral transmission-factor 
for any illimiinant, the spectral characteristics of other 



CERTAIN PHYSICAL ASPECTS AND DATA 



395 



thicknesses, and those of combinations of these thick- 
nesses with other colored glasses as already outlined. 

For the sake of further exemplification, in Fig. 148 
are shown the straight-line relations between thickness 
and transmission-factor (for entering radiation) for 
several wave-lengths for various thicknesses of a gold 
ruby glass. The relations between luminosity and 



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Fig. 150. — Test of * straight-line » law. 

thickness for this glass are shown in Fig. 149 for various 
wave-lengths. Fig. 148 is a diagram of what would be 
seen if the solid represented in Fig. 141 were viewed 
from the right-hand side. 

At this point it is of interest to show the approximation 
of experimental results to the relations between spectral 
character and thickness as predicted by theory. This 
is shown in Fig. 150 for one of the glasses made during 
the development of an * artificial-daylight ' glass some 
years ago. The method of graphical analysis was tested 
because of the desire for a simple method and the speci- 



396 COLOR AND ITS APPLICATIONS 

men was ground and polished in five thicknesses. The 
circles show the verification of the theory. In this case 
correction had not been made for surface reflection 
so the straight lines must be drawn to a point near 0.92 
on the transmission-axis. Incidentally, it is of interest 
to note that previous to the adoption of this method, 
samples of melts were groimd in the form of thin wedges 
and spectral analyses were made at various thicknesses. 
It is seen that the graphical method enormously reduces 
the amount of work in order to obtain the data neces- 
sary for such studies. 

In developing a colored glass for a specific purpose, 
various factors are considered such as the illuminant 
to be used and the result to be obtained. From these 
an ideal spectral transmission-curve is determined and 
by means of a few spectral analyses of different colored 
glasses, bearing in mind the chemical considerations 
if a mixture is finally necessary, various combinations 
can be made with the aid of the graphical method. 

Often the tdtra-violet and infra-red spectral trans- 
missions are of interest and these are made in the man- 
ner already described. The data on a coloring element 
is not considered to be sufl^ciently complete for record 
if the ultra-violet transmission is not studied at least 
qualitatively and in some cases the infra-red transmis- 
sion is investigated. 

106. Influence of Temperature on Transmission of 
Colored Glasses. — Hyde, Cady, and Forsythe ^ in study- 
ing red pyrometer-glasses, noted the influence of temper- 
ature on the transndssion characteristic of a red glass 
and investigated this influence for temperatures from 20° 
to 80° C. The transmission-factor of the red glass was 
foimd to be appreciably less for various ^yave-lengths 
at the higher temperatures than at the lower temper- 
atures. 



CERTAIN PHYSICAL ASPECTS AND DATA 397 

It appeared of interest to ascertain how generally 
the transmission-factors of colored glasses were affected 
by temperature.^^ In a preliminary study it did not 
appear worth while to investigate this question spectro- 
photometrically; therefore, only the transmission-factor 
for total visible radiation was considered. However, 
an idea of the change in spectral transmission is gained 
through the change in color of the specimen as its temper- 
ature is altered. It is hoped that at a later date a careful 
study of this phenomenon can be made spectrophoto- 
metrically and in parallel with chemical investigations. 

In order to eliminate the annoyance of large color- 
differences in determining the transmission-factors at 
different temperatures, a given specimen was cut into 
two pieces and one was kept at a temperature of 30° C, 
while the temperature of the other was altered gradually 
from this temperature to 350° C. The transmission- 
factor of a colored glass, of course, varies with the 
illuminant so that such a value is indefinite unless the 
illuminant is specified. In this accotmt it appears suffi- 
cient to state that the illuminants used were gas-filled 
Mazda lamps operating at normal voltage. A continuous 
check on the constancy of the light-sources and of the 
transmission-factors of the optical paths was made 
possible by removing the two colored glasses from the 
optical paths momentarily without altering the temper- 
ature conditions. The relative transmission-factors of 
the two pieces of the given specimen were measured 
throughout the range of temperature indicated and the 
results for ten commercial specimens are given in the 
diagram and in Table XXXI. 

No color-difference was encountered during the meas- 
urements except that due to a change in the spectral 
transmission characteristic of the heated specimen. 
This color-difference became very marked for specimens 



398 



COLOR AND ITS APPLICATIONS 



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CERTAIN PHYSICAL ASPECTS AND DATA 399 

5 and 10. The transmission-factor of the hotter piece 
is given in terms of that of the colder piece of the same 
specimen at the various temperatures — that is, the 
transmission-factors as given are relative values and not 
absolute. The color of a specimen at the highest temper- 
ature is given as compared with that of a piece of the 
same specimen at 30° C, the change being sufficient to 
be readily described in terms of our ordinary indefinite 
terminology. All the glasses excepting the two contain- 
ing cobalt decreased in transmission-factor as the tem- 
perature increased, and in some cases this decrease 
in transmission-factor was very large. The curves ob- 
tained by plotting temperature and relative transmission- 
factor are, in general, approximately straight lines in- 
dicating that throughout this range of temperature the 
transmission-factor changes approximately proportionally 
with the temperature for the specimens used. Owing to 
the relatively slight change in hue in the red end of the 
spectrum, the red glasses 1 and 9 did not change ap- 
preciably in color when heated, notwithstanding large 
decreases in their transmission-factors. 

This preliminary study indicates an interesting field 
for careful research which might throw more light upon 
the question, *How are glasses colored?' It will be 
noted that the highest temperature studied is below 
that at which the glass becomes self-luminous or plastic. 
It will be of interest to carry this investigation close to 
the melting point. The results obtained are of interest 
both theoretically and practically. 

107. Ultra-violet Transmission. — Another interesting 
fact reported by the author " is the increase in trans- 
mission of clear glass for certain ultra-violet rays by 
the addition of cobalt. In other words, the range of 
transmission extended further in the ultra-violet region 
in the case of the cobalt glass than in the case of the 



400 COLOR AND ITS APPLICATIONS 

clear glass, the slight amount of cobalt in the former 
being the only difference in the compositions of the two 
glasses. 

Absolam ^^ has presented the data in Table XXXII 
which indicate the wave-lengths where complete absorp- 
tion commences; that is, in each case the wave-length 
indicating the longest one of the region of practically 
complete absorption. He used an arc between copper 
poles and a quartz spectrograph. 
TABLE xxxn 

Natural blue rock salt Beyond 225/>i/i 

Natural rock salt colored by cathode rays " 225 

Natural rock salt colored blue by cathode rays " 225 

Sylvite white " 225 

Chile saltpetre, ordinary white variety 351 

" " violet 325 

Fluorspar, colored deep violet by cathode rays Beyond 225 

Diamond, yellow 320 

Diamond, blue 315 

Kimzite 305 

Garnet 402 

Zircon, (hyacinth) red-brown 262 

Zircon, decolorized by heat 244 

Zircon, green • 402 

Zircon, yellow 402 

Topaz, pale yellow . 262 

Topaz, dark yellow 229 

Topaz, pale pink-brown 262 

Topaz, blue 296 

Emerald 320 

Ruby 300 

Tourmaline, green 351 

*• green-yellow 300 

" pmk 306 

spinel, blue 402 

" purple 325 

" pmk 300 

Kyanite, blue 320 

Beiyle, blue 327 

Cordierite, blue-purple 325 

Cairngorm 325 

Ordinary clear glass is practically opaque beyond 
300/xju although clear glasses vary considerably in trans- 



CERTAIN PHYSICAL ASPECTS AND DATA 401 

parency to ultra-violet depending upon the content of 
silica and other ingredients. In general, the color of a 
substance is no indication of its transparency to invisible 
radiation. 

108. Compounds Sensitive to Temperature. — Experi- 
ence with the effect of temperature on colored glasses 
leads to the belief that the same effect would be found 
with pigments and solutions. In fact, some of these 
effects have been noticed and it would be of interest 
to investigate this point systematically. Certain com- 
pounds change color with change in temperature and 
they are of practical as well as of scientific interest. 

The double iodide of mercury and silver is normally 
a light yellow but its color changes to a deep orange or 
red at about 50° C. Its color will return to normal on 
cooling unless it has been overheated. It is prepared 
from separate aqueous solutions of silver nitrate and 
potassium iodide. The latter is added to the former 
until the original precipitate is dissolved. At this point 
a strong solution of mercuric chloride is added and the 
precipitate formed is the bright yellow double iodide of 
mercury and silver. This is filtered, washed, and dried. 
It may be used as a paint by mixing into a solution of 
gum arable. 

The double iodide of mercury and copper is normally 
red but changes to black at about 85° C. returning in 
color to red as it cools. This is prepared in a manner 
similar to the compovmd in the preceding paragraph 
excepting that copper sulphate is substituted for silver 
nitrate. 

109. Transmission of Water and Fog. — The se- 
lective scattering and consequent selective absorption 
of the atmosphere is well known and is illustrated in 
Fig. 13. The fine particles of dust and even molecules 
of gas are responsible for scattering the rays of shorter 



402 COLOR AND ITS APPLICATIONS 

wave-length more than those of longer wave-length. 
For this reason the setting sun is red ; a cloud of smoke 
is blue, and the shadow of a puff of smoke is brownish. 
That this selectivity is dependent upon the size of the 
particles is also apparent. For example, smoke from 
the tip of a cigar is more bluish than that emanating 
from the moist end. In the latter case moisture has 
condensed around the carbon nuclei and these larger 
particles do not scatter light so selectively. 

It is also known that water appears various tints of 
blue and blue-green when it is of great depth and purity. 
This is especially noticeable when flying over bodies of 
water although the effect of the color of the bottom 
and of the suspended matter washed from the shore 
must be separated from that of the water alone. Or- 
dinary observations indicate that water selectively 
transmits rays in the green region; that is, rays of 
wave-lengths near the ends of the visible spectrum are 
transmitted less freely than those near the middle or 
especially in the green region— between 0.5/x and 0.6/^. 
This seems to have been fairly well established by 
experiment. 

Recently Utterback ^^ made some determinations of 
the passage of various colored lights, obtained by means 
of filters, through artificial fogs produced by expanding 
saturated air. His results indicate that his fogs were 
most transparent to light rays of wave-lengths from 
0.53/x to 0.59m. The transparency decreased rapidly 
toward the red but not so rapidly toward the blue end 
of the spectrum. Abbott ^^ obtained similar results 
for water-vapor when there were dust particles in the 
air. 

Bertel using a * submarine' spectrograph photo- 
graphed the visible spectrum of the light reaching various 
depths. His results show the visible spectrum to be 



CERTAIN PHYSICAL ASPECTS AND DATA 



403 



rapidly narrowed. The red rays being totally absorbed 
at depths of 5 to 10 meters; the orange at 20 meters; 
and the yellow at 100 meters. In the other end of the 
spectrum Httle selective absorption appears to take place 
until a depth of 30 meters is reached. At 1700 meters 
no light has been detected by any investigator. The 
range of the spectrograms obtained at various depths 
are presented in Table XXXIII. 



TABLE 


XXXTTI 




Deptiis in meters 


Range of ' 


wrave-lengths 


2 


ZOSfXfi 


to lOOjJLIX 


10 


303 


618 


20 


306 


688 


30 


310 


677 


^ 60 


322 


668 


338 


561 


60 


346 


556 


70 


350 


660 


80 


355 


647 


90 


367 


546 


100 


360 


643 


200 


379 


513 


300 


392 


600 


400 


398 


498 



Many factors can influence the results obtained so 
that there is bound to be disagreement. However, 
water appears to have a definite selective transmission 
for light of a hue in the neighborhood of green. 

110. Color-temperature of lUuminants. — The vari- 
ous colorimeters and the spectrophotometer have been 
used for the purpose of comparing illuminants and of 
representing their spectral characteristics respectively. 
Another method is to compare the integral color of an 
illuminant (at normal operation of the lamp) with the 
color of a * black-body' radiator and rating the former 
in terms of the temperature of the latter when a color- 



404 COLOR AND ITS APPLICATIONS 

match (approximate in some cases) obtains. In Table 
XXXIV the results obtained by Hyde and Forsythe ^^ 
are presented in terms of absolute temperatures (Kel- 
vin scale). These values are termed * color temper- 
atures.' From these data and those on the black-body 
brightness-temperatures, the mean brightness of each 
light-source may be computed. 

TABLE XXXIV 

Gas flame, fish-tail (coal and water-gas) 1870 deg. K. 

Hefner 1875 

Pentane, 10 c. p. standard 1914 

Candle, paraflBn 1920 

Candle, sperm 1925 

Kerosene, cylindrical wick 1915 

Kerosene, flat wick 2045 

Acetylene, as a whole 2368 

Acetylene, one spot 2448 

Nemst glower, 2.3 w. p. h. c 2388 

Carbon filament, 4.0 w. p. h. c 2070 

Treated carbon filament, 3.1 w. p. h. c 2153 

Metallized carbon filament, 2.5 w. p. h. c. 2183 

Osmium filament, 2.0 w. p. h. c 2176 

Tantalum filament, 2.0 w. p. h. c 2249 

Tungsten filament, 1.25 w. p. h. c 2385 

Tungsten filament, 0.9 w. p. h. c 2543 

Tungsten filament (gas-filled), 0.6 w. p. h. c 2900 (approx.) 

Kingsbury ^^ using some of the foregoing values as 
reference points has made measurements of the color- 
temperature of commercial gas-burners obtaining values 
from 1940 to 2118 deg. K, As is to be expected, the 
color-temperature of a flame is within certain limits 
dependent upon its shape, size, and position and upon 
the composition of the gas. This method of rating 
illuminants yields valuable results. 

REFERENCES 

1. Phil. Trans, of Roy. See, A, 203, p. 385. 

2. Bull. Bur. Stds., 9, p. 283. 

3. Proc. Amer. See. Test. Mat., 1916, 17, part H. 



CERTAIN PHYSICAL ASPECTS AND DATA 405 

4. Hyde, Forsythe, and Cady, Astrophys. Jour., 1918, 48, p. 65. 

5. Elec. Wld., May 19, 1917. 

Jour. Frank. Inst., 1918, 186, p. 52Sf. 
Jour. Opt. Soc, 1919, 2-3, p. 39. 

6. Trans. Amer. Electrochem. Soc, 1916. 

7. Astrophys. Jour., 1915, 42, p. 285. 

8. Astrophys. Jour., 1918, 47, p. 83. 

9. Astrophys. Jour., 1915, 42, p. 302. 

10. Jour. Amer. Ceramic Soc, 1919, 2, p. 743. 

11. Jour. Frank. Inst, 1918, 186, p. 111. 

12. Phil. Mag., 1917, 33, p. 452. 

13. Gen, Elec. Rev., 1917, 20, p. 671. 

14. Trans. I.E.S., 1919. 

15. Annals of Astrophys. Obs., 3, p. 214. 

16. Jour. Frank. Inst., 1917, 183, p. 353. 

17. Jour. Frank. Inst., 1917, 183, p. 781. 



! 



INDEX TO AUTHORS 



Abney, 39, 68, 85, 93, 96, 104, 109, 190, 

297 
Aitken, 326 
Alcmaeon, 181 
Anaxagoras, 181 
Aristotle, 181 
Arons, 107 
Ashe, 132 
Aubert, 127, 143, 190 

Babbage, 226 

Baily, 223 

BaltzeU, 318 

Becquerel, 214 

BeU, 130, 196 

Benham, 40 

Bloch, 106, 144 

Blondel and Rey, 144 

BoU, 187 

Bradford, 262, 320, 326 

Broca and Sulzer, 137, 144 

Brown, 223 

Briicke, 177 

Burch, 180 

Busstyn, 148 

Byk, 223 

Cady, 20 

Charpentier, 144 

Chevreul, 68, 79, 176, 307, 311 

ChurchiU, 147, 151 

Clutsam, 326 

Cobb, 123, 131 

Cohn, 262, 320, 326 

Crookes, 159 

Cros, 218 

Crova, 159, 197, 229 

Dember, 212 
Democritus, 181 
Diogenes, 181 
Bonders, 186 



Dow, 132, 203 

Dufton and Gardner, 229, 241 

Ebbinghaus, 189 

Ebet, 326 

Edridge-Green, 124, 178, 188, 190 

Empedocles, 181 

Eimer, 103 

Fabry, 107, 197 

Fechner, 39, 121 

Ferree, 208 

Ferry, 143 

Fery and Cheneveau, 197 

Fick, 143 

Fraunhofer, 18 

Gamett, 37 
Geissler, 127, 326 
Greenwood, 189 

Hagen, 85 

HaU, 343 

Harrison, 285 

Hauron, du, 218, 223 

Haycraft, 146 

Helmholtz, 40, 116, 143, 171, 172, 177, 

180 
Hering, 172, 177, 178, 184 
Hertz, 6 
Hewitt, 44 
Holmgren, 151 
Houston, 199 
Hughes, 326 
Hussey, 229, 242 
Hyde, E. P., 20, 90, 114, 143 
Hyde and Forsythe, 212 
Hyde, F. S., 343 

Ives, F. E., 102, 218, 221, 242 
Ives, H. E., 20, 93, 103, 131, 146, 196, 
197, 204, 209, 214, 217, 229, 238, 274, 
281, 285, 301, 305, 314, 323 



407 



408 



INDEX TO AUTHORS 






Ives and Brady, 111, 245 
Ives and Kingsbury, 198, 212 
Ives and Luckiesh, 127, 202, 242 

Javel, 226 

Johnson, 223 

Joly, 60, 217, 218 

Jones, B., 258 

Jones, L. A., 96, 98, 127, 237 

Jorgensen, 301 

Karrer, 199 

Kingsbury, 212 

Kirchhoflf, 14 

Kirchhoff and Bunsen, 107 

Klein, 180 

Kleiner, 143 

Knoblauch, 46 

Koenig, 11, 101, 102, 181, 189, 199, 210 

Koenig and Brodhun, 120 

Koenig and Martens, 113 

Koenig, £., 223 

Koettgen, 229 

Kries, v., 146, 183 

Kiihne, 187 

Ladd-Franklin, 186 

Lambert, 66 

Lea, 213 

Lehmann, 214 

Lippmann, 30, 214 

Loeser, 132 

Lucas, 197 

Luckiesh, 20, 63, 76, 88, 99, 130, 133, 
138, 143, 146, 162, 164, 168, 196, 
205, 207, 229, 239, 243, 246, 256, 260 

Luckiesh and Cady, 91, 229, 238 

Lumiere, 60, 219 

Lummer and Brodhun, 88, 108, 143 

MacDonald, 326 

MacDougal, 163 

Major, 326 

Martel, 301 

MaxweU, 6, 61, 73, 101, 124 

Mayer, 178 

Mees, 62, 229 

MerriU, 245 

Mie, 37 



Millar, 204 
Miller, 226 
Moore, 241, 268 
Morris-Airey, 207 
Mott, 343 
Munsell, 79 

Nagel, 180, 189 

Nemst, 197 

Neuhaus, 214 

Newton, 23 

Nicati, 190 

Nichols, 228 

Nichols and Franklin, 229 

Nichols and Merritt, 212 

Nicol, 32 

Nutting, 22, 88, 94, 96, 112, 121, 127, 



Ostwald, 301 

Paget, 219 

Parry and Coste, 343 

Parsons, 169, 190 

Paterson, 307, 311 

Paterson and Dudding, 148 

Pfund, 212 

Pirani, 229 

Planck, 14 

Plateau, 143 

Plato, 181 

Porter, 146 

Prang, 82 

Preston, 22 

Priest, 212 

Purkmje, 11, 164, 191, 204 

Rasch, 197 

Rayleigh, 37, 124, 197 
Rice, 133 
Richtmeyer, 212 
Ridgeway, 85, 124 
Rimington, 312, 316, 322, 326 
Rood, 40, 68, 85 
Rowland, 29 
Ruchmich, 326 
Runge, 78 
Ruxton, 82 
Ryan, 257 



INDEX TO AUTHORS 



409 



Schanz and Stockhausen, 169 

Schwartzchild, 202 

Scriabine, 316 

Seebeck, 214 

Seig and Brown, 212 

Sharp and Millar, 243 

Shepherd, 221 

Simmance and Abady, 64 

SneUen, 131 

Stammer, 107 

Starcke, 223 

Stebbins, 212 

Steindler, 125 

Stefan-Boltzmann, 16 

Stevenson, 77 

Stuhr, 204 

Talbot, 143 

Thomson, 37 

Thorp, 217 

Titchener, 76, 262, 320, 326 

Toch, 343 

Torda, 212 

Townsend, 212 

Tschermak, 180 

TyndaU, 37 



Uhler and Vood, 49 
Uhthoff, 133 

Valenta, 214 
Vanderpoel, 301 
Vinci, da, 177 
Voege, 169 
V6gel, 216, 229 



"Weideman and Messerschmidt, 143 

Wien-Paschen, 14 

Wiener, 213, 214 

Whitman, 100 

Wmch, 326 

Wollaston, 18 

Wood, 22, 47, 216, 291 

Wimdt, 190 

Young, 26, 181 

Young-Hehnholtz, 74, 101, 103, 172, 
181, 186 

Zenker, 214 
Zimmerman, 62 
Zindler, 86 



INDEX OF SUBJECTS 



Aberration, chromatic, 118, 284 
Abney's templates, 110 
Absorption, 35 

by atmosphere, 147 

by dust, smoke, 304 

selective, 36 

spectra, 50, 51 
atlas of, 62 

of solid dyes and refractive index, 
309 

of rhodamine reflector, 45 
Acetic acid, 334 
Acetone, 334 

Acetylene, spectrum of, 21 
Achromatic lens, 119 * 

Acid violet, 306 
Additive disks, papers for making, 63 

method of mixing colors, 57 

primary colors, 57 
Advertising, colored light in, 278 

displays, 274 
Aesculine, 43, 202 

fluorescence of, 43 

absorption of ultraviolet by, 43 
Affective value of colors, 262 
After-images, 170, 284 

colored, 171 

complementary, 170 

duration of, 171 

effect of, 170 

explanation of, 172 

in painting, 282 

negative, 170 

positive, 170 

production of, 170 
Air brush, 342 
Alcohol, 333 
Allegheny Coimty Soldiers' Memorial, 

257 
Amber, 336 

glass, 264 



Amyl acetate, 334 

alcohol, 334 
Analysis of color, 86 
colored media, 96 
color of illimiinants 

by photometer and color 

filters, 107 
by colored solutions, 107 
by monochromatic colori- 
meter, 97 
by polarization colorimeter, 

108 
by trichromatic colorimeter, 
105 
Aniline dyes, 303, 328 

reflection from solid, 309 
powdered, 309 
Aniline yellow, 57 
Anthracene, 43 

Appearance of colors affected by 
duration of stimulus, 163 
environment, 163, 307 
illuminant, 163, 286, 303, Plate IV 
intensity of illmnination, 163 
size and position of retinal image, 

163 
surface character, 163 
retinal adaptation, 163 
mercury arc, 166 
Arc spectrum, 17, 21 
Art and light, 285 
Art galleries, 258, 294 

artificial daylight in, 244 
Artificial daylight, 238 
and the colorist, 305 
production of, 227, 230, 235 
uses for, 234 
testing, 306 

versus natural light, 226, 227 
Artist, aim of, 282 
attitude of, 282 



411 



412 



INDEX TO SUBJECTS 



Artist, position of, 282 

photography and the, 283 
terminology used by the, 284 

Artists' pigments, 328 

Art of mobile color, 312 

Atmospheric absorption, 147 

Auramine, 340 

Atirantia, 331 

Average daylight, 228 

Balmain's paint, 43 
Banana oil, 334 
Benham disk, 39 
Benzene, 334 
Benzine, 334 
Binocular contrast, 177 
Black, absolute, 72 
Black body, 14 

radiation, 21 
Black, bone, 333 

ivory, 333 

lamp, 333 

nigrosine, 333 
Black paper, 72 

velvet, 72 
Blue, cobalt, 298, 329 

Prussian, 329 

ultramarine, 298, 329 
Blue-green, filter, 57 
Bone black, 333 
Booth, demonstration, 266 
Borax bead, 309 
Brightness, spectral sensibility and, 10 

contrast, 174, Plate III 

of colors, 70 

effect of illuminant on, 168 
Brightness increment, 122 

scale, 81 

sensibility of retina, 122 
Brush, air, 342 

Cadmium yellow, 298, 331 

Calcixmi fluoride, 41 

Canada balsam, 335 

Carbon dioxide tube, Moore, 241 

incandescent lamp spectrum, 21 
Carmine, 298, 332 
Cascade method, 208 



Celluloid, uses of, 339 

coloring, 340 
Changeable colors, 309 
Charts, color, 82 
Chlorophyl, 43, 310 
Chrysoidine, 340 
Chromatic aberration, 284 

of eye, 118 
Chrome yellow, 331 
Chromium oxide, 298, 330 
Chromoscope, 218 
Clouds, selective transmission of, 

38 
Cobalt blue, 298, 329 

glass, 205 
Collodion, 336 
Colloidal solutions, 37 
Color analysis, 86 

of illuminants, 97, 105, 107, 108 

of media, 96 
Color and light, 23 

and vision, 117 

blindness, tests for, 151 
Color box. Maxwell, 161 

chart. Prang, 82 
Ruxton, 82 

codes, 317 

cylinder, Chevreul, 79 

effects, disappearing and chan- 
ging, 275, 278 
for stage and displays, 272 
modem tendencies in, 276 
principle of, 272 
spectacular, 257 
Color harmony, 251 

in decoration, 251, 267 

in glasses, 37 

in interiors, 251 j 

in lighting, 224 

in north rooms, 251 

in south rooms, 252 

matching, 302 
glasses, 308 
light, 309 
Color-mixing apparatus, 60 

disk, 64 
Color mixtiu-e, 54 
Color music, 314 

suggested in Nature, 319 



INDEX TO SUBJECTS 



413 



Color names, 78 
notation, 77 

of sun altitude, 38 , 

phenomena in painting, 282 
Color photography, 213 
Lippmann, 214 

Wood diffraction process, 215 
filter processes of, 218 
Joly, 218 
Paget, 219 
Lumiere, 219 
Shepherd, 221 
Ives, 221 
Kinemacolor, 222 
Kodachrome, 222 
Color photometry, 191 
preference, 260, 320 
production of, 23 
pyramid, 75 
sensation curves, 104 
sensations, growth and decay of, 
137, 164 
produced by colorless stimtili, 
39 
Color sphere, Runge, 78 
terminology, 69 
tree, Mimsell, 79 
triangle, 73 
vision theories, 181 
wheel, 59 
Colored fabrics, appearance of wet, 310 
gelatine, 327 
glasses, 327 

analysis of, 97 
for eliminating glare, 154 
for protection against ultra- 
violet, 157 
for use with field glasses, 160 
for varying contrast, 160 
in the industries, 159 
tests of, 157 
uses for, 151 
Colored headlights, 152 
lacquers, 327 
light in hoine, 252, 259 
lights and colored objects, 273 
lights, range of, 148 
Colored media, analysis of, 96 
papers, 328 



Colored papers, reflection co-efficients 
of, 168 
imder colored light, 273 
Zimmerman, 63 
Colored patterns, successive contrast 
and, 173 

photographs, projection of, 218 

shadows, 269 

surrotmdings, effect of, 227, 245, 
250 
Colorimeter, monochromatic, 93 

trichromatic, 101 

analysis of illtuninants by, 97, 105, 
107, 242 
Coloring materials, 294, 327 
Colorless stimuli, color sensations 

from, 39 
Colors, 283 

affective value of, 260, 320 

and soimds, 312 

artistic, 262 

changeable, 309 

cool, 252 

emotive value of, 260, 320 

examination of, 307 

Fechner, 39 

for demonstration, 306 

in Nature, 35, 54 

monochromatic, 35, 167 

of feathers, 30 

of fiery opals, 28, 30 

of insects, 30 

of potassium chlorate, 30 

pigment, 35 

produced by mixing pigments, 56 

purity of spectral, 35 

two-component mixttu-es of, 99 

used with music, 317 

warm, 252 
Complementary colors, 66 

filters, 57 

hues, 59 

spectral hues, 59, 76 
Cones, retinal, 120 
Congressional Ubrary, 257 
Continuous spectra, 16 
Contrast, binocular, 177 • 

brightness, 174 

hue, 174 



414 



INDEX TO SUBJECTS 



Contrast, in Nature, 291 

in pigments, 291 

in paintings, 292 

simultaneous, 174, 285, Plate III 

successive, 173 

theories of simultaneous, 177 
Copal, 335 
Critical frequency. Porter's law of, 145 

wave form and, 146 
Crova's method of photometry, 197 

solution, 197 
Crown glass, 86 
Crystals, 30, 32 
Cyan blue, 221 
Cyanine, 37, 303, 306 
Cylinder, color, 79 

Dammar, 336 

Daylight, artificial, 227, 230, 235, 305 

average, 228 

color of, 38 

efficiency of illuminants, 231, 233 

testing artificial, 306 

uses for artificial, 234 

variability of, 228, 304 

versus artificial light, 225, 227 
Decoration, color in, 251, 257 
Defects of color photography, 220 
Defining power of eye, 283 
Demonstration booth, 266 
Dichroic dyes, 303, 306, 308 
Dichroism, 37 
Diffraction 26 

color photography, 216 

grating, 26, 29 
copies of, 29 
Rowland's, 29 
spectnun, Plate I 
Direct-comparison photometry, 203 
Disk, Benham, 39 

for varying brightness, 71 

for varjdng saturation, 71 

Maxwell, 61 

sectored, 90 

Whitman, 100 
Dispersion of glass, 25 

prismatic, 23 
Displays, 274 
Distribution of light on paintings, 291 



Durability of pigments, 342 
Dyes, aniline, 328 
^uorescent, 310 

Ear, analytic ability of, 313 

comparison of eye and, 313 
Edridge-Green theory, 187 
Effect, PurMnje, 11 
Effects of radiant energy, 7 
Efficiency, daylight, 233 

lighting, 228 

radiant, 13 
Electromagnetic theory of light, 6 
Electron, 6 

Emerald green, 298, 330 
Environment, and colors, 303 
Emotive value of colors, 320 
Eosme, 310 

pink, 303 
Equality-of-brightness photometry, 203 
Erythrosine, 306 
Ether, 5, 334 
Ethyl alcohol, 334 

violet, 37, 57, 63 
Extraordinary ray, 33 
Eye, 116 

a synthetic instrument, 314 

chromatic aberration in, 118 

as a simple lens, 283 

compared with ear, 314 

faults of, 283 
. not analytic, 92, 313 

movements, 284 

optical constants of, 117 

Fabry's solutions, 198 
Feathers, color of, 28, 30 
Fechner coefficient, 121 

colors, 39 

law, 121 
Fibers, transparency of, 303 
Film, celluloid, 339 

gelatine, 338 
Filters, complementary, 57 

for panchromatic plate, 202 

for ultraviolet bands, 47, 51 

for visible rays, 47 

useful, 46 
Flashing sign, novel, 279 



INDEX TO SUBJECTS 



415 



Flicker photometer, Whitman-disk, 100 
Simmance-Abady, 64 

photometry, 203 
Flickering Ughts, 139 
FUnt glass, 86 
Fluorescein, 43, 310 
Fluorescence, 41 

colors and, 42 

effect of solvent on, 45 

examination of, 41 

excitation of, 42 

in color matching, 308, 310 

tests of, 310 
Fluorescent dyes, 310 

reflector, 44, 153 

media, 43 
Fluorite prism, 26 
Fluor spar, 41 
Fovea centralis, 184, 307 
Fraunhofer lines, 18, 19, Plate I 
Frosting solution, 337 

Gamboge, 298, 331 
Gelatine, 327, 334 

filters, 269, 337 
Glass, color of, 37 

crown, 86 

dispersion of, 25 

flint, 86 

prism, 26 

transmission of, 26 
Glasses, colored, 154, 327 
Grain alcohol, 334 
Grating, diffraction, 26 

spectnun, Plate I 
Green made by mixing yellow and blue, 

299, 330 
Growth of color sensations, 137, 142, 

164, 207 
Gum kauri, 336 
Gum water, 295 

Hauron color photography, 218 
Headlights, green-yellow, 152 
Hefner lamp, spectnun of, 21 
Hering theory, 184 
Heterochromatic photometry, 191 
Holmgren test, 151 
Houston's solutions, 199 



Hue, 70 

and the illuminant, 169, 286, Plate 
IV 

contrast, 176, Plate IH 

difference, minimum, 125 

sensibility, 124 
Huyghen's principle, 5 

Iceland spar, 32 

Dluminants, brightnesses of colors and, 
167 

misuse of, 226 

simulating old, 253 

spectra of, 13 

temperature and color of, 13 

values and, 167 
Elusion of intense illvunination, 291 
Impressionism, 60 
Indian red, 332 

yellow, 298, 331 
Indigo, 298, 330 
Induction, 175 
Infra-red, opacity of water to, 42 

photography, 47 
Insects, color of, 30 
Interference, 29 

constructive, 3 

destructive, 3 
Interiors, color in, 251 
Iridescent crystals, 28, 29 
Irradiation, 179 
Isolating spectral lines, 47, 51 
Ives, (F. E.) color photography, 221 
Ivory black, 333 

Joly color photography, 218 
Juxtapositional method, 60 

Kerosene, 43 

Kinemacolor, 222 

Kodachrome color photography, 222 

Kries (v.) duplicity theory, 183 

Lacquers, 336 

celluloid, 337 

colored, 327 
Ladd-Franklin theory, 186 
Lakes, 332 



416 



INDEX TO SUBJECTS 



Lambert color-mixer, 65 
Lamp black, 333 
Laws of radiation, 14 
Law, Bloch, 144 

Blondel and Rey, 144 

Porter, 145 

Talbot, 143 
Legibility of type, 137 . 
Lens, achromatic, 119 

simple, 118 
Light and Art, 285 

color, 23 
Light beam, diagram of, 31 
Light, 1 

definition of, 1 

electromagnetic theory of, 6 

production, 12 

sensation, 7 

shade, and color, 282 

the soul of art, 285 

velocity of, 6 

waves, 4 

analogies of, 3 

and sound waves, 313 

white, 9, 38 
Lights of short duration, 143 
Lighting artist, 285 

color in, 224 

of art galleries, 258 

of paintings, 286, 291 
Line spectra, 16 
Linseed oil, 334 

Lippmann color photography, 214 
Lumiere color photography, 219 
Luminosity curve of eye, 208 

equation for, 211 

Macula lutea, 307 
Madder pigments, 332 
Magenta, 221 
Malachite green, 307, 331 
Martins yellow, 331 
Mastic, 335 
Matching of colors, 302 

artificial daylight for, 305 
Maxwell disks, 61 

color triangle, 73 

color box, 101 
Mercury arc, spectrum of, 17, 46, 50 



Mercury arc, visual acuity and, 131, 
136 

colors imder, 166 
Methods of color photometry, 192, 208 

limitations of, 193 

secondary, 196 
Methyl alcohol, 333 

violet, 37, 303, 306 
Mica, 29 

Miscellaneous notes, 341 
Mixture of colors, 54 

by shadows, 66 

two-component, 99 
Mobile-color art, 312 

development of, 317 

future of, 326 

instrmnents for, 321 
Monochromatic colors, 35, 167 

acuity in, 135 
Moore tube, 241 
Multiple reflection, 36, 248, 308 
Music, development of, 312 

evolution of, 318 
Musical notation, 78 

Naphthol green, 57, 63, 202 

yellow, 306, 331 
Naphthalin red, 310 
Neodymium, 47 
Newton's experiment, 23 

rings, 30 
Nicol prism, 33 
Nigrosine, 307, 333 
Non-selective brightness control, 114 
Normal spectrum, 26, Plate I 
Notation, color, 77 
Novel color effects, 274 

Ochres, 332 

Old Ulimiinants, simulating, 253 
Opal, fiery, 28, 30 
solution, 337 
Oil fihn, 29 
Ordinary ray, 33 
Organic dyes, 43 
Overhand method, 310 

Painting, after-images in, 173 
color phenomena in, 282 



INDEX TO SUBJECTS 



417 



Painting, artificial daylight for, 286 

in artificial light, 287 
Paintings, cleaning, 296 

hanging, 292 

lighting, 291, 294 
Paints, 294, 329 

phosphorescent, 341 
Panama-Pacific Exposition, 257 
Papers, colored, 328 

yellow vs. white, 226 
Paraffin prism, 26 
Phloxine, 310 
Phosphorescence, 41, 340 
Photo-electric cell, 196, 200 
Photography, color, 231 

infra-red, 47 

the artist and, 283 

true values in, 201 

ultraviolet, 47 
Photometry, color, 191, 207 

filters for, 108 

primary methods of, 192 

secondary methods of, 196 
Pigments, 169, 294, 328 

characteristics of, 298 

classes of, 296 

contrast by, 291 

durabihty of, 297, 342 

limitations of, 291 

mixing, 66, 297 

purity of, 297, 299 

sources of, 296 
Pitch prism, 26 
Planck's law, 14 
Plane of polarization, 31 

rotation of, 34 
Plane-polarized Ught, 31 
Polarization, 30, 31 

by crystals, 32 

by reflection, 31 
Polarized light, 31 
Poppy oil, 334 

Potassium bichromate, 306, 331 
Preference, color, 260, 320 
Primary colors, 56, 57 
Primary sensation curves, 182 
Printing inks, 328 
Prismatic spectrum, 18, Plate I 
Prisms, 26 



Production of Ught, 12 
Prussian blue, 265, 330, 340 
Purity of colors, 70 
Purkinje effect, 11, 164, 191, 204 

reversed, 205 
Purple, 74, 167 

visual, 187 
Pyramid, color, 76, 76 

Quartz, dispersion of, 26 
polarization by, 32 
prism, 26 
transparency, 26 

Radiant efficiency, 13 

energy, 7 
Radiation and light sensation, 7 

and temperature, 11 

from a solid, 8 

laws, 14 
Rainbow, 7, 24 
Range of colored lights, 148 
Red, 332 

References, 22, 53, 68, 86, 114, 161, 
180, 189, 211, 223, 270, 281, 301, 311, 
326, 343 
Reflection, selective, 36 
Reflectometer, 112 
Refraction, 23 
Refractive index, 25 

absorption of dyes and, 309 
Resins, 335 

solubility of, 336 
Resorcin-blue, 310 
Retina, brightness sensibility of, 122 

color sensibility of, 119, 307 
Retinal rivalry, 177 
Rhodamine, 202, 303, 306, 310 

reflector, 44 
Rivalry, retinal, 177 
Rock salt prism, 26 
Rods, 119 
Rose bengal, 310 
Rotation of plane of polarization, 34 

Sandarac, 336 
Saturation of colors, 70 

sensibility, 127 
Scattered light, 37 



418 



INDEX TO SUBJECTS 



Scattered light, colored glasses and, 

162 
Sectored disk, 90, 114 
Seeing, 282 
Selective absorption, 35, 38 

reflection, 28, 248 

scattering, 38 

transmission, 36, 38 
Sensation curves, primary, 182 
Sensibility, brightness, 122 

hue, 119, 124 

retinal, 120 

saturation, 127 
Shades, 71 
Shadows, colored, 66 

daylight, 304 

in painting, 291 
Shellac, 336 

Shepherd color photography, 221 
Shooting glasses, 164 
Signaling, 146 

Ughts for, 146, 162 
Silver fihn, 48 

Simmance-Abady photometer, 64 
Simultaneous [contrast, 174, Plate 

m 

instantaneity of, 178 

in color matching, 307 

in painting, 286 
Skylight, color of, 38 

origin of, 38 

spectnun of, 21 

natural, 304 

artificial, 306 
Slit of spectroscope, 24 
Smoke, absorption by, 38 
Soap bubbles, 30 
Solar spectrum, 17, 18 
Solutions, Crova, 197 

Fabry, 198 

Houston, 199 

Ives and Kingsbury, 198 

Karrer, 199 
Solvents, 333 
Sotmds and colors, 312 
Spectra, arc, 17 

of gases, 16 

of illuminants, 13, 20, 21 

representative, 17 



Spectra, of solids, 16 

ultraviolet, 60, 61 
Spectral character, influence of, 167, 
286 

colors, 35 

complementaries, 76 

distribution of energy, 20, 21 

lines, 19 

sensibility of eye, 10 

transmission of media, 91 
Spectrophotometer, 69, 88 

simple, 92 

portable, 89 
Spectroscope, 86 

direct vision, 86 

accessories for, 87 

comparison, 88 
Spectrum analysis, 16 
Spectnmi of daylight, 17 

helitmi, 17 

mercury, 17, 46, 60 

of soditmi, 48 

of tungsten, 17 
Spectrum, energy, 8 

grating, 26, Plate I 

normal, 26, Plate I 

production of, 24 

rotating colored disk, 68 

visible, 8 

total, 8 
Specular reflection, 309 
Sphere, color, 78 
Spherical light waves, 6, 26 
Stage, color effects for, 272 
Standardization of colors, 84 
Standing wave, 3 
Stefan-Boltzmann law, 15 
Subtractive disks, 63 

color-mixing, 54, 298 

primary colors, 66 
Subjective yellow, 48 
Successive contrast, 173 
Sunlight, 38, 304 

artificial, 239, 306 
Surface character, influence of, 36, 

169, 302 
Surface color, 309 

Surroimdings, influence of, 246, 304, 
Plate m 



I. 



INDEX TO SUBJECTS 



419 



Talbot's law, 143 

Tartrazine, 202, 331 

Temperature, color of light and, 9, 13 

radiation and, 11 

spectrum and, 9 
Templates, 109 
Terminology, 69 
Terra verte, 298, 330 
Theory of color vision, 181 

Edridge-Green, 187 

Hering, 184 

V. Kries, 183 

Ladd-Franklin, 186 

Young-Hehnholtz, 181 
Thinner, 295 
Tints, 71 
Tourmaline, 32 
Transmission, 35 

glass, 26 

selective, 36 

quartz, 26 
Tree, color, 79 
Triangle, color, 73, 76 
Tri-color method, 73 
Timgsten lamps, spectrum of, 21 
Turpentine, 335 

Venice, 336 

Ultramarme blue, 265, 298, 329 
Ultraviolet transmission of media, 50 

spectra, 50 
Uramn, 43, 57, 310 
Uranium glass, 42 
Uviol blue glass, 42 

Value scale, 81 
Values, 70, 283 

illuminants and, 167, 286 

lighting and, 286 
Varnish, 295, 335 
Vehicles, 295 
Velocity of light, 6 
Venetian red, 332 



Venice turpentine, 335 
Vermilion, 298, 332 
Visibility of radiation, 209 

of point sources, 149 
Vision, 278 

color and, 116 
Visual acuity in colored hght, 129, 136 

field, 120 

luminosity filter, 199 

phenomena in painting, 282, 284 
in color matching, 302 
Visual purple, 187 

bleaching, 188 

extracting, 177 
Visual yellow, 188 

Wall covering for paintings, 294 
Wave motion, 2 

analogies of, 3, 5 
Wave theory, 1 

Welsbach mantle, spectriun of, 21 
Wheel, color, 59 
White lead, 333 
White hght, 9, 38 

aesthetic, 265 

artificial, 304 

standard, 303 

subjective, 55, 235 
Wien-Paschen law, 15 
Wood alcohol, 333 
Wood color photography, 215 
Wtmdt colored papers, 63 

Yellow pigments, 331 

solutions, 48 

spot, 307 

visual, 188 

versus white paper, 226 
Young-Hebnholtz theory, 101, 181 
Young's double sUt expt., 26 

Zinc chromate, 331 
white, 333 



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Binnie, Sir A. Rainfall Reservoirs and Water Supply 8vo, 

Binns, C. F. Manual of Practical Potting 8vo, 

The Potter's Craft i2mo, 

Birchmore, W. H. Interpretation of Gas Analysis i2mo, 

Blaine, R. G. The Calculus and Its Applications lamo, 

Blanchard, W. M. Laboratory Exercises in General Chemistry. .i2mo, 

Blasdale, W. C. Quantitative Chemical Analysis ». i2mo, 

Bloch, L. Science of Illumination 8vo, 

Blyth, A. W. Foods: Their Composition and Analysis 8vo, 

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Bockmann, F. Celluloid i2mo, 

Bodmer, G. R. Hydraulic Motors and Turbines i2mo, 

Boileau, J. T. Traverse Tables 8vo, 

Bonney, G. E. The Electro-platers' Handbook i2mo, 

Boone, W. T. A Complete Course of . Volumetric Analysis i2mo, 

Booth, N. Guide to the Ring-spinning Frame i2mo, 



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D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 

Booth, W. H. Water Softening and Treatment 8vo, 

Superheaters and Superheating and Their Control . . . 8vo, 

Bottcher, A. Cranes: Their Construction, Mechanical Equipment and 

Working 4to (Reprinting.)' 

Bottler, M. Modern Bleacbing Agents. i2mo, 

Bottone, S. R. Magnetos for Automobilists i2mo, 

Electro-Motors, How Made and How Use i2mo, 

Boulton, S. B. Preservation of Timber i6mo, 

Bourcart, E. Insecticides, Fungicides and Weedkillers 8vo, 

Bourgougnon, A. Physical Problems i6mo, 

Bourry, E. Treatise on Ceramic Industries 8vo, 

Bowie, A. J., Jr. A Practical Treatise on Hydraulic Mining 8vo, 

Bowls, 0. Tables of Common Rocks i6mo, 

Bowser, E. A. Elementary Treatise on Analytic Geometry i2mo, 

Elementary Treatise on the Differential and Integral Calculus. i2mo, 

Elementary Treatise on Analytic Mechanics i2mo, 

Elementary Treatise on Hydro-mechanics i2mo, 

A Treatise on Roofs and Bridges i2mo. 

Boycott, G. W. M. Compressed Air Work and Diving Svo, 

Bradford, G. Whys and Wherefores of Navigation. i2mo, 

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Bragg, E. M. Design of Marine Engines and Auxiliaries Svo, 4 00 

Brainard, F. R. The Sextant i6mo, 

Brassey's Naval Annual for 1919 Svo, 10 00 

Briggs, R., and Wolff, A. R. Steam-Keating i6mo, o 75 

Bright, C. The Life Story of Sir Charles Tilsoj Bright Svo, *4 50 

Telegraphy, Aeronautics and War Svo, 6 00 

Brislee, T. J. Introduction to the Study- of Fuel Svo, 3 50 

Broadfoot, S. K. Motors: Secondary Batteries i2mo, o 75 

Broughton, H. H. Electric Cranes and Hoists 

Brown, G. Healthy Foundations i6mo, o 75 

Brown, H. Irrigation , Svo, 6 00 

Brown, H. Rubber Svo, 2> 50 

W. A. Portland Cement Industry Svo, 3 00 

Brown, Wm. N. Dipping, Burnishing, Lacquering and Bronzing 

Brass Ware i2mo, *i 50 

Handbook on Japanning i2mo, *2 00 

Brown, Wm. N. The Art of Enamelling on Metal i2mo, *2 00 

House Decorating and Painting i2mo, *2 00 

History of Decorative Art i2mo *o 50 

Workshop Wrinkles Svo, *i 00 

Browne, C. L. Fitting and Erecting of Engines Svo, *i 50 

Browne, R. E. Water Meters i6mo, o 75 

Bruce, E. M. Detection of Common Food Adulterants lamo, i 40 

Brunner, R. Manufact\ire of Lubricants, Shoe Polishes and Leather 

Dressings Svo, 3 50 

Buel, R. H. Safety Valves i6mo, o 75 

Bunkley, J. W. Military and Naval Recognition Book i6mo, i oc 

hurley, G. W. Lathes. Their Construction and Operation i2mo, 2 00 

. Machine and Fitting Shop Practice. 2 vols i2mo, each, 2 00 

Testing of Machine Tools lamo, 2 00 

Burnside, W. Bridge Foundations lamo, *2 00 



6 D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 

Biirstall, F. W, Energy Diagram for Gas. With Text 8vo, 150 

Diagram. Sold separately *i q^ 

Burt, W. A. Key to the Solar Compass i6mo, leather, 250 

Buskett, E. W. Fire Assaying i2mo, *i 25 

Butler, H. J, Motor Bodies and Chassis 8vo, *3 00 

Byers, H. G., and Knight, H. G. Notes on Qualitative Analysis 8vo, 

{New Edition in Preparation.) 



Cain, W. Brief Course in the Calculus < n n • < < n i 1 . . . . I2m0) 

Elastic Arches i6mo, 

Maximum Stresses i6mo, 

Practical Designing Retaining of Walls i6mo, 

■ Theory of Steel-concrete Arches and of Vaulted Structures. 

i6mo, 

' Theory of Voussoir Arches i6mo, 

■ Symbolic Algebra i6mo, 

Calvert, G. T. The Manufacture of Sulphate of Ammonia and 

Crude Ammonia i2mo, 

Camm, S.; Aeroplane Construction i2mo, 

Carhart, H. S. Thermo Electromotive Force in Electric Cells. . .i2mo, 

Carey, A. E., and Oliver, F. W. Tidal Lands. 8vo, 

Carpenter, t . D. Geographical Surveying , i6mo, 

Carpenter, R. C, and Diederichs, H. Internal Combustion Engines. 8vo, 

Caxpmaei, H. lillectric Welding and Welding Appliances 4to, 

Carter, H. A. Ramie (Rhea), China Grass i2mo, 

Carter, H. R. Modern Flax, Hemp, and Jute Spinning 8vo, 

Bleaching, Dyeing and Finishing of Fabrics 8vo, 

Cary, E. R. Solution of Railroad Problems with the Slide Rule.i6mo, 
Casler, M. D. Simplified Reinforced Concrete Mathematics. .. .i2mo, 

Cathcart, W. L. Machine Design. Part I. Fastenings 8vo, 

Cathcart, W. L., and Chaffee, J. I. Elements of Graphic Statics . . .8vo, 

— — Short Course in Graphics i2mo, 

Caven, R. M., and Lander, G. D. Systematic Inorganic Chemi6try.i2mo, 

Chalkley, A. P. Diesel Engines 8vo, 

Chalmers, T. W. The Production and Treatment of Vegetable Oils, 

4to, 

Paper Making and its Machinery 4to, 

■ The Gyroscopic Compass 8vo, 

Chambers' Mathematical Tables 8vo, 

Chambers, G. F. Astronomy i6mo 

Chappel, E. Five Figure Mathematical Tables 8vo, 

Charnock, Mechanical Technology 8vo, 

Charpentier, P. Timber 8vo, 

ChatJey, H. Principles and Designs of Aeroplanes i6mo, 

' How to Use Water Power i2mo, 

'• Gyrostatic Balancing 8vo, 

Child, C. D. Electric Arc 8vo, 

Christian, M. Disinfection and Disinfectants i2mo, 

Christie, W. W. Boiler-waters, Scale, Corrosion, Foaming Svo, 

' Chimney Design and Theory Svo, 

• Furnace Draft i6mo, 

Water: Its Purification and Use in the Industries Svo, 

Church's Laboratory Guide 8vo, 



^i 75 

o 75 
o 75 
o 75 

o 75 
o 75 

75 

4 00 
3 00 
2 CO 

5 00 

5 50 

5 00 

*3 00 

*3 50 

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1 25 
I 25 

*3 50 
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1 50 

2 25 

5 00 

7 50 

8 00 
5 00 
2 50 
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*3 00 

o 75 

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2 50 



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Cisin, H. J. Modem Marine Engineering lamo (/n Press.) 

Clapham, J, H. Woolen and Worsted Industries 8vo, 200 

Clapperton, G. Practical Papermaking .8vo {Reprinting.) 

Clark, A. G. Motor Car Engineering. ^ 

Vol. I. Construction *4 00 

Vol.11. Design 8vo, *3 50 

Clark, C. H. Marine Gas Engines. New Edition 2 00 

Clarke, J. W., and Scott, W. Plumbing Practice. 

Vol. I. Lead Working and Plumbers' Materials 8vo, *., 00 

Vol. n. Sanitary Plumbing and Fittings (/« Press.) 

Vol. III. Practical Lead Working on Roofs {In Press.) 

Clarkson, R. P. Elementary Electrical Engineering i2mo, 

Clerk, D., and Idell, F. E. Theory of the Gas Engine i6mo, 

Clevenger, S. R. Treatise on the Method of Government Surveying. 

i6mo, morocco, 

Clouth, F. Rubber, Gutta-Percha, and Balata 8vo, 

Cochran, J. Concrete and Reinforced Concrete Specifications Svo, 

Treatise on Cement Specifications Svo, 

Cocking, W. C. Calculations for Steel-Frame Structures i2mo. 

Coffin, J. H. C. Navigation and Nautical Astronomy i2mo, 

Colbum, Z., and Thurston, R. H. Steam Boiler Explosions. .. .i6mo. 

Cole, R. S. Treatise on Photographic Optics i2mo, 

Coles-Finch, W. Water, Its Origin and Use Svo, 

Collins, C. D. Drafting Room Methods, Standards and Forms Svo, 

Collins, S. Hoare. Plant Products and Chemical Fertilizers Svo, 

Chemical Fertilizers Svo, 

Collis, A. G. High and Low Tension Switch-Gear Design Svo, 

Switchgear i2mo, 

Colver, E. D. S. High Explosives Svo, 

Comstock, D. F., and Troland, L. T. The Nature of Electricity and 
Matter Svo, 

Coombs, H. A. Gear Teeth i6mo, 

Cooper, W. R. Primary Batteries Svo, 

Copperthwaite, W. C. Tuimel Shields 4to, 

Corfield, W. H. Dwelling Houses i6mo, 

Water and Water-Supply i6mo, 

Cornwall, H. B. Manual of Blow-pipe Analysis Svo. 

Couch, J. F. Dictionary of Chemical Terms i2mo, fabrikoid, 

Cowee, G. A. Practical Safety Methods and Devices ...Svo, 

Cowell, W. B. Pure Air, Ozone, and Water i2mo, 

Craig, J. W., and Woodward, W. P. Questions and Answers About 

Electrical Apparatus i2mo, leather, 

Craig, T. Motion of a Solid in a Fuel i6mo, 

Wave and Vortex Motion i6mo, 

Crehore, A. C. Mystery of Matter and Energy Svo, 

The Atom i2mo, 

Crocker, F. B., and Arendt, M. Electric Motors Svo, 

Crocker, F. B., and Wheeler, S. S, The Management of Electrical Ma- 
chinery i2mo, *i 00 

Crosby, E. U., Fiske, H. A., and Forster, H. W. Handbook of Fire 

Protection i2mo, 4 00 

Cross, C. F., Bevan, E. J., and Sindall, R. W. Wood Pulp and Its 

Uses Svo, 350 

Cros»key, L. R. Elementary Perspective Svo, i 50 



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8 D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 

Crosskey, L. R., and Thaw, J. Advanced Perspective 8vo, 2 00 

CuUey, J. L. Theory of Arches i6mo, o 75 

Cushing, H. C, Jr., and Harrison, N. Central Station Management. .. *2 00 

Dadourian, H. M. Analytical Mechanics. i2mo, 3 75 

Graphic Statics 8vo, o 75 

Danby, A. Katural Rock Asphalts and Bitumens 8vo, *2 50 

Darling, E. R. Inorganic Chemical Synonyms lamo, i 00 

Davenport, C. The Book 8vo, 250 

Davey, N. The Gas Turbine Svo, 

Davies, F. H. Electric Power and Traction Svo, 

Davis, A. M. Introduction to Palaeontology Svo, 

— • — Foundations and Machinery Fixing i6mo, 

Deerr, N. Sugar Cane Svo (Reprinting.) 

Deite, C. Manual of Toilet Soap-Making Svo, 

De la Coux, H. The Industrial Uses of Water Svo, 

Del Mar, W. A. Electric Power Conductors Svo, 

Denny, G. A. Deep-level Mines of the Rand 4to, 

De Roos, J. D. C. Linkages i6mo, 

Derr, W. L. Block Signal Operation Oblong i2mo, 

Desaint, A. Three Hundred Shades and How to Mix Them Svo, 

De Varona^ A. Sewer Gases i6mo, 

Devey, R. G. Mill and Factory Wiring i2mo, 

Dichmann, Carl. Basic Open Hearth Steel Process i2mo, 

Dieterich, K. Analysis of Resins, Balsams, and Gum Resins .... Svo, 

Dilworth, E. C. Steel Railway Bridges 4to, 

Dinger, Lieut. H. C. Care and Operation of Naval Machinery. . .i2mo, *3 oo 
Dixon, D. B. Machinist's and Steam Engineer's Practical Calculator. 

i6mo, morocco, i 25 
Dommett, W. E. Motor Car Mechanism. i2mo, *2 00 

Dorr, B. F. The Surveyor's Guide and Pocket Table-book. 

i6mo, morocco, 2 00 

Draper, C. H. Heat and the Principles of Thermo-Dynamics. .i2mo, 225 

Draper, E. G. Navigating the Ship lamo, 2 00 

Dubbel, H. High Power Gag Engines Svo, *5 00 

Dumesny, P., and Noyer, J. Wood Products, Distillates, and Extracts. 

Svo, *5 00 
Duncan, W. G., and Penman, D. The Electrical Equipment of Collieries. 

Svo, *5 00 

Dunkley, W. G. Design of Machine Elements. Two volumes. .Svo,each, 2 00 

Dimstan, A. E., and Thole, F. B. T. Textbook of Practical Chemistry. 

i2mo, 

Durham, H. W. Saws Svo, 

Duthie, A. L. Decorative Glass Processes Svo, 

Dwight, H. B. Transmission Line Formulas Svo, 

Dyke, A. L. Dyke's Automobile and Gasoline Engine Encyclopedia, 

Svo, 

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Dyson, S. S., and Clarkson, S. S. Chemical Works Svo, 

Eccles, W. H. Wireless Telegraphy and Telephony i2mo, 7 00 



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E^, J. Light, Radiation and Illumination 8vo, 

Eddy, L. C. Laboratory Manual of Alternating Currents i2mo, 

Edelman, P. Inventions and Patents i2mo, 

Edgcumbe, K. Industrial Electrical Measuring Instruments 8vo, 

Edler, R. Switches and Switchgear 8vo, 

Eissler, M. The Metalliurgy of Gold 8vo, 

— — The Metallurgy of Silver 8vo, 

The Metallurgy of Argentiferous Lead 8vo, 

A Handbook on Modern Explosives 8vo, 

EMn, T. C. Water Pipe and Sewage Discharge Diagrams folio, 

Electric Light Carbons, Manufacture of 8vo, 

Eliot^ C. W., and Storer, F. H. Compendious Manual of Qualitative 

Chemical Analysis lamo, 

Eliott, A. W. M. Rectangular Areas i2mo, 

Ellis, C. Hydrogenation of Oils 8vo, 

Ultraviolet Light, Its Applications in Chemical Arts 1 2mo, 

(In Press) 

and Meigs, J. V. Gasolene and Other Motor Fuels., (/m Press.) 

Ellis, G. Modern Technical Drawing 8vo, 

Ennis, Wm. D. Linseed Oil and Other Seed Oils 8vo, 

Applied Thermodynamics 8vo, 

Vapors for Heat Engines i2mo, 

Ennenf W. F. A. Materials Used in Sizing 8vo, 

Erwin, M. The Universe and the Atom i2mo (Reprinting.) 

Ewing, A. J. Magnetic Induction in Iron 8vo, 5 00 

Page, A. Airscrews in Theory and Practice 4to, 

Fairchild, J. F. Graphical Compass Conversion Chart and Tables... 

Faille, J. Notes on Lead Ores i2mo, 

Notes on Pottery Clays i2mo, 

Fairley, W., and Andre, Geo. J. Ventilation of Coal Mines. .. .i6mo, 

Fairweather, W. C. Foreign and Colonial Patent Laws 8vo, 

Falk, K. G. Chemical Reaction^: Their Supply and Mechanism. .i2mo, 

Fanning, J. T. Hydraulic and Water-supply Engineering 8vo, 

Famsworth, P. V. Shop Mathematics i2mo (In Press.) 

Fay, L W. The Coal-tar Dyes 8vo, 

Fembach, R. L. Glue and Gelatine 8vo, 

Findlay, A. The Treasures of Coal Tar i2mo, 

Firth, J. B. Practical Physical Chemistry i2mo, 

Fwichet, E. The Preparation of Organic Compounds i2mo, 

Fisher, H. K. C, and Darby, W. C. Submarine Cable Testing. . .8vo, 

Fleischmann, W. The Book of the Dairy 8vo, 

Fleming, J. A. The Alternate-current Transformer. Two Volumes. 8vo. 

Vol. I. The Induction of Electric Currents *6 50 

Vol, II. The Utilization of Induced Currents 6 50 

Propagation of Electric Currents 8vo, 500 

A Handbook for the Electrical Laboratory and Testing Room. Two 

Volumes 8vo, each, *6 50 

Flenry, P. Preparation and Uses of White Zinc Paints 8vo, 3 00 

Flynn, P. J. Flow of Water i2mo, o 75 

Hydraulic Tables i6mo, o 75 



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Foster, H. A. Electrical Engineers' Pocket-book. {Seventh Edition.) 

i2mo, leather, 5 00 

Engineering Valtiation of Public Utilities and Factories . 8vo, *3 00 

Fowle, F. F. Overhead Transmission Line Crossings i2mo, *i 5a 

The Solution of Alternating Current Problems Cvo {In Press.) 

Fox, W. G. Transiti«n Curves i6mo, o 75 

Fox, W., and Thomas, C. W. Practical Course in Mechanical Draw- 
ing i2mo, 

Foye, J. C. Chemical Problems i6mo, 

< Handbook of Mineralogy i6mo, 

Francis, J. B. Lowell Hydraulic Experiments 4to, 

Franzen, H. Exercises in Gas Analysis i2mo, 

Fraser, E. S., and Jones, R. B. Motor Vehicles and Their Motors, 

8vo, fabrikoid, 

Freudemacher, P. W* Electric Mining Installations i2mo, 

Friend, J. N. The Chemistry of Linseed Oil i2mo, 

Fritsch, J. Manufacture of Chemical Manures 8vo, 

Frye, A. I. Civil Engineers' Pocket-book i2mo, leather, 

Fuller, G. W. Investigations into the Purification of the Ohio River. 

4to, *] 
Pumell, J. Paints, Colors, Oils, and Varnishes 8vo. 

Gant, L. W. Elements of Electric Traction 8vo, 

Garcia, A. J. R. V. Spanish-English Railway Terms 8vo, 

Gardner, H. A. Paint Researches, and Their Practical Applications, 

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Fires i2mo, leather, 

Garrard, C. C. Electric Switch and Controlling Gear Bvo, 

Gaudard, J. Foundations i6mo, 

Gear, H. B., and Williams, P. F. Electric Central Station Distribution 

Systems Bvo, 

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Chemical Control m Cane Sugar Factories 4to, 

Geikie, J. Structural and Field Geology Bvo, 

Mountains. Their Growth, Origin and Decay Bvo, 

The Antiquity of Man in Europe Bvo, 

Georgi, F., and Schubert, A. Sheet Metal Working Bvo, 

Gerhard, W. P. Sanitation, Watersupply and Sewage Disposal of Country 

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Household Wastes i6mo, 

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Gerhardi, C. W. H. Electricity Meters Bvo, 

Geschwind, L. Manufacture of Alum and Sulphates Bvo, 

Gibbings, A. H. Oil Fuel Equipment for Locomotives. Bvo. 

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Gibson, A. H. Hydraulics and Its Application Bvo, 

. Water Hammer in Hydraulic Pipe Lines i2mo, 

Gibson, A. H., and Ritchie, E. G. Circular Arc Bow Girder 4to, 



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D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 11 

Gilbreth, F. B. Motion Study i2mo, 

Primer of Scientific Management i2ma, 

Gill A. H. Gas Analysis for Chemists 8vo, 

Gillmore, Gen. Q. A. Roads, Streets, and Pavements i2mo, 

Godfrey, E. Tables for Structural Engineers i6mo, leather, 

Golding, H. A. The Theta-Phi Diagram i2mo, 

Goldschmidt, R. Alternating Current Commutator Motor 8vo, 

Goodchild, W. Precious Stones 8vo, 

Goodell, J. M, The Location, Construction and Maintenance of 

Roads 8vo, 

Goodeve, T. M. Textbook on the Steam-engine i2mo, 

Gore, G. Electrolytic Separation of Metals Bvo, 

Gould, E. S. Arithmetic of the Steam-engine T2mo, 

Calculus i6mo, 

High Masonry Dams i6mo, 

Gould, E. S. Practical Hydrostaticsi and Hydrostatic Formulas. .i6mo, 

Goulding, E. Cotton and Other Vegetable Fibres 8vo, 

Gratacap, L. P. A Popular Guide to Minerals 8vo, 

Gray, H. H. Gas- Works Products Bvo (In Press.) 

Gray, J. Electrical Influence Machines i2mo, 2 00 

Marine Boiler Design i2mo (Reprinting.) 

Greenhill, G. Dynamics of Mechanical Flight 8vo, 

Greenwood, H. C. The Industrial Gases 8vo, 

Gregorius, R. Mineral Waxes i2mo, 

Grierson, R. Some Modern Methods of Ventilation 8vo, 

Griffiths, A. B. A Treatise on Manures i2mo (Reprinting.) 

Gross, E. Hops 8vo, 

Grossman, J. Ammonia and Its Compounds i2mo, 

Groth, L, A. Welding and Cutting Metals by Gases or Electricity. 

Bvo, 

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Gruner, A. Power-loom Weaving 8vo, 

Grunsky, C. E. Topographic Stadia Surveying i6mo, 

Gunther, C. 0. Integration Bvo, 

Gurden, R. L. Traverse Tables folio, 

Guy, A. E. Experiments on the Flexure of Beams, Bvo, 

Haenig, A. Emery and Emery Industry Bvo, 

Hainbach, R. Pottery Decoration i2mo, 

Hale, A. J. The Manufacture of Chemicals by Electrolysis 8vo, 

Hale, Harrison. American Chemistry i2mo (In Press.) 

Hale, W. J. Calculations of General Chemistry i2mo. 

Hall, C. H. Chemistry of Paints and Paint Vehicles i2mo, 

Hall, R. H. Governors and Governing Mechanism i2mo. 

Hall, W. S. Elements of th« Differential and Integral Calculus Bvo, 

— - Descriptive Geometry 8vo volume and a 4to atlas, 

Haller, G. F., and Cunningham, E. T. The Tesla Coil i2mo, *i 25 

Halsey, F. A. Slide Valve Gears. , i2mo, 1 50 

The Use of the Slide Rules i6mo, 075 

Worm and Spiral Gearing i6rao, o 75 

Hamlin, M. L. Action of Chemicals on Industrial Materials. .Bvo, 

(In Press.) 



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12 D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 



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*2 


00 



Hancock, H. Textbook of Mechanics and Hydrostatics. 8vo, i 

Hardy, E, Elementary Principles of Graphic Statics i2mo, *i 

Haring, H. Engineering Law. 

Vol. I. Law of Contract 8vo, *4 00 

Harper, J. H. Hydraulic Tables on the Flow of Water i6mo, *2 00 

Harris, S. M. Practical Topographical Surveying {In Press.) 

Harrow, B. Eminent Chemists of Our Times: Their Lives and Work. 2 50 

• From Newton to Einstein i2mo, 

Harvey, A. Practical Leather Chemistry 8vo, 

Haskins, C. H. The Galvanometer and Its Uses i6mo, 

Hatt, J. A. H. The Colorist square i2mo, 

Hausbrand, E. Drying by Means of Air and Steam izmo, 

Evaporating, Condensing and Cooling Apparatus 8vo, 

Hausmann, E. Telegraph Engineering Svo, 

Hausner, A. Manufacture of Preserved Foods and Sweetmeats. .. .Svo, 
Hawkesworth, J. Graphical Handbook for Reinforced Concrete Design. 

4to, 

Hay, A. Continuous Current Engineering Svo, 

Hayes, H. V. Public Utilities, Their Cost New and Depreciation. . .Svo, 

• Public Utilities, Their Fair Present Value and Return Svo, 

Heath, F. H. Chemistry of Photography Svo. (In Press.) 

Heather, H. J. S. Electrical Engineering Svo, 

Heaviside, 0. Electromagnetic Theory. Vols. I and II.... Svo, each, 

{Reprinting.) 

' Vol. Ill Svo {Reprinting.)- 

Heck, R. C. H. The Steam Engine and the Turbine Svo, 

• Steam-Engine and Other Steam Motors. Two Volumes. 

Vol. I. Thermodynamics and the Mechanics Svo, 

Vol. II. Form, Construction, and Working Svo, 

Notes on Elementary Kinematics Svo, boards, 

Graphics of Machine Forces Svo, boards, 

Heermann, P. Dyers' Materials i2mo, 

Hellot, Macquer and D'Apligny. Art of Dyeing Wool, Silk and Cotton. Svo, 
Hering, C, and Getman, F. H. Standard Tables of Electro-Chemical 

Equivalents i2mo, 

Hering, D. W. Essentials of Physics for College Students Svo, 

Herington, C. F. Powdered Coal as Fuel Svo, 

Herrmann, G. The Graphical Statics of Mechanism i2mo, 

Herzfeld, J. Testing of Yarns and Textile Fabrics Svo. 

Hildenbrand, B. W. Cable-^Making i6mo, 

Hilditch, T. P. A Concise History of Chemistry i2mo. 

Hill, M. J. M. The Theory of Proportion Svo, 

Hillhouse, P. A. Ship Stability and Trim Svo, 

Hiroi, I. Plate Girder Construction i6mo, 

' Statically-Indeterminate Stresses i2mo, 

Hirshfeld, C. F. Engineering Thermodynamics i6mo, 

Hoar, A. The Submarine Torpedo Boat i2mo. 

Hobart, H. M. Heavy Electrical Engineering .Svo, 

Design of Static Transformers i2mo, 

Electricity 8vo, *2 00 

Electric Trains Svo {Reprinting.) 

Electric Propulsion of Ships Svo, *2 50 



^2 00 



4 50 



4 50 

4 50 

5 50 
I 00 
I 00 
3 00 



'2 00 



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D. VAN NOSTRAND CO/S SHORT TITLE CATALOG 13 

Hobart, J. F. Hard Soldering, Soft Soldering and Brazing lamo, i 25 

Hobbs, W. R. P. The Arithmeric of Electrical Measurements i2mo, o 75 

Hoff, J. N. Paint and Varnish Facts and Formulas i2mo, 2 00 

Hole, W. The Distribution of Gas 8vo, *8 50 

Holmes, A. Nomenclature of Petrology 8vo, 5 00 

Hopkins, N. M. Model Engines- and Small Boats i2mo, i 25 

The Outlook for Research and Invention '. i2mo. 2 00 

Hopkinson, J., Shoolbred, J. N., and Day, R. E. Dynamic Electricity. 

i6mo. o 7 a 

Horner, J. Practical Ironfounding 8vo, *2 00 

Gear Cutting, in Theory and Practice 8vo (Reprinting.) 

Houghton, C. E. The Elements of Mechanics of Materials i2mo, 

Houstoun, R. A. Studies in Light Production i2mo, 

Hovenden, F. Practical Mathematics for Young Engineers lamo, 

Howe, G. Mathematics for the Practical Man i2mo, 

Eov/orth, J. Repairing and Riveting Glass, China and Earthenware. 

'' 8vo, paper, 

Hoyt, W. E. Chemistry by Experimentation 8vo, 

Plubbard, E. The Utilization of Wood-waste 8vo, 

Hiibner, J. Bleaching and Dyeing of Vegetable and Fibrous Materials. 

8vo, 

Hudson, 0. F. Iron and Steel 8vo, 

Humphreys, A. C. The Business Features of Engineering Practice . 8vo, 

Hunter, A. Bridge Work 8vo. (In Press.) 

Hurst, G. H. Handbook of the Theory of Color ..8vo, 

Dictionary of Chemicals and Raw Products Svo, 

Lubricating Oils, Fats and Greases Svo, 

Soaps Svo, 

Hurst, G. H., and Simmons, W. H. Textile Soaps and Oils Svo, 

Hurst, H. E., and Lattey, R. T. Text-book of Physics Svo, 

Also published in three parts. 

Part I. Dynamics and Heat 2 00 

Part II. Sound and Light 2 00 

Part III. Magnetism and Electricity 2 00 

Hutchinson, R. W.j Jr. Long Distance Electric Power Transmission. 

i2mo, 3 00 

Hutchinson, R. W., Jr., and Thomas, W. A. Electricity in Mining. i2mo. 

(In Press.) 

Hyde, E. W. Skew Arches. i6mo. o 75 

Hyde, F. S. Solvents, Oils, Gums, Waxes Svo, *2 00 

Induction Coils i6mo. o 75 

Ingham, A. E. Gearing. A practical treatise Svo, *2 50 

Ingle, H. Manual of Agricultural Chemistry Svo, 500 

Inness, C. H. Problems in Machine Design x2mo, *3 00 

Centrifugal Pumps i2mo, *3 00 

The Fan , lamo, *4 00 

Jacob, A, and Gould, E. S. On the Designing and Construction of 

Storage Reservoirs i6mo. o 75 



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14 D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 

Jacobs, F. B. Cam Design and Manufacture ( In Press.) 

James, F. D. Controllers for Electric Motors. , 8vo. <» oo 

Jehl, F. Manufacture of Carbons 8vo, «; oo 

Jennings, A. S. Commercial Paints and Painting 8vo. 2 «;o 

Jennison, F. H. The Manufacture of Lake Pigments 8vo, 6 00 

Jepson, G. Cams and the Principles of their Construction .8vo, *i 50 

Mechanical Drawing 8vo (In Preparation.) 

Jervis-Smith, F. J. Dynamometers 8vo. 4 00 

Jockin, W- Arithmetic of the Gold and Silversmith i2mo, *i 00 

Johnson, C. H., and Earle, R. P. Practical Tests for the Electrical 

Laboratory ( In Press.) 

Johnson, J. H. Arc Lamps and Accessory Apparatus i2mo, o 75 

Johnson, T. M. Ship Wiring and Fitting i2mo {Reprinting.) 

Johnston, J. F. W., and Cameron, C. Elements of Agricultural Chemistry 

and Geology i2mo, 2 60 

Joly, J. Radioactivity and Geology i2mo (Reprinting.) 

Jones, H. C. Electrical Nature of Matter and Radioactivity lamo, 

Nature of Solution 8vo, 

New Era in Chemistry i2mo, 

Jones, J. H. Tinplate Industry 8vo 

Jones, M. W. Testing Raw Materials Used in Paint i2mo, 

Jordan, L. C. Practical Railway Spiral i2mo, leather, 

Jiiptner, H. F. V. Siderclogy: The Science of Iron 8vo, 

Itapp, G. Alternate Current Machinery ...,»».. i6mo, 

Xapper, F. Overhead Transmission Lines 4to, 

Keim, A. W. Prevention of Dampness in Buildings. 8vo, 

Keller, S. S., and Knox, "W. E. Analytical Geometry and Calculus... 
Kemble, W. T., and Underhill, C. R. The Periodic Law and the Hydrogen 

Spectrum 8vo, paper, 

Kemp, J. F. Handbook of Rocks 8vo, 

Kennedy, A. B. W., and Thurston, R. H. Kinematics of Machinery. 

i6mo, 
Kennedy, A. B, W., Unwin, W. C, and Idell, F. E.' Compressed Air. 

i6mo, 

Kennedy, R. Flying Machines ; Practice and Design i2mo, 

Principles of Aeroplane Construction 8vo, 

Kent, W. Strength of Materials. i6mo, 

Kershaw, J. B. C. Fuel, Water and Gas Analysis 8vo, 

Electrometallurgy 8vo, 

Electro-Thermal Methods of Iron and Steel Production 8vo, 

The Use of Low Grade and Waste Fuel for Power Generation . 8vo, 

Kingzett, C. T. Popular Chemical Dictionary 8vo, 

Kinzbrunner, C. Continuous Current Armatures 8vo, 

Testing of Alternating Current Machines Svo, 

Kinzer, H., and Walter, K. Theory and Practice of Damask Weaving, 

8vo, 4 00 
Kirkaldy, A.. W., and Evans, A. D. History and Economics of 

Transport 8vo, *3 00 

^rbride, J. Engraving for Illustration Svo, *i 00 



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50 


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D- VAN NOSTRAND CO.'S SHORT TITLE CATALOG 15 

Eirschke, A. Gas and Oil Engines lamo, *i 50 

Klein, J. F. Design of a High-speed Steam-engine 8vo, 

Physical Significance of Entropy 8vo, 

Klingenberg, G. Large Electric Power Stations 4to, 

Knight, R.-Adm. A. M. Modern Seamanship 8vo, 

Pocket Edition i2mo, f abrikoid, 

Knott, C. G., and Mackay, J. S. Practical Mathematics 8vo, 

Knox, J. Physico-Chemical Calculations i2mo, 

Fixation of Atmospheric Nitrogen i2mo, 

Koester, F. Steam-Electric Power Plants 4to, 

Hydroelectric Developments and Engineering 4to, 

Koller, T. The Utilization of Waste Products Bvo, 

Cosmetics Bvo, 

Koppe, S. W. Glycerine i2mo, 

Kozmin, P. A. Flour Milling 8vo, 

Krauch, C. Chemical Reagents 8vo, 

Kremann, R. Application of the Physico-Chemical Theory to Tech- 
nical Process and Manufacturing Methods ...8vo, 3 00 

Kretchmar, K. Yarn and Warp Sizing 8vo, *5 00 

Laffargue, A. Attack in Trench Warfare i6mo, 

Lallier, E. V. Elementary Manual of the Steam Engine i2mo, 

Lambert, T. Lead and Its Compounds 8vo, 

Bone Products and Manures Bvo, 

Lamborn, L. L. Cottonseed Products Bvo, 

Modern Soaps, Candles, and Glycerin 8vo, 

Lamprecht, R. Recovery Work After Pit Fires 8vo, 

Lanchester, F. W. Aerial Flight. Two Volumes. Bvo. 

VoL I. Aerodynamics *6 qo 

Vol. II. Aerodonetics *6 

Lanchester, F. W. The Flying Machine 8vo, 

Industrial Engineering: Present and Post-War Outlook. . .i2mo, 

Lange, K. R. By-Products of Coal-Gas Manufacture i2mo. 

La Rue, B. F. Swing Bridges i6mo, 

Lassar-Cohn, Dr. Modern Scientific Chemistry i2mo, 

Latimer, L. H., Field, C. J., and Howell, J. W. Incandescent Elect-ric 

Lighting i6mo, 

Latta, M. N. Handbook of American Gas-Engineering Practice. .Bvo, 

American Producer Gas Practice , .4to, 

Laws, B. C. Stability and Equilibrium of Floating Bodies: .... .Bvo, 

Lawson, W. R. British Railways. A Financial and Commercial 

Survey 8vo, 

Leask, A. R. Refrigerating Machinery i2mo (Reprinting.) 

Lecky, S. T. S. "Wrinkles" in Practical Navigation Bvo, 

Pocket Edition i2mo, 

Danger Angle i6mo, 

Le Doux, M. Ice-Making Machines i6mo, 

Leeds, C. C. Mechanical Drawing for Trade Schools oblong 4to, 

jirechanical Drawing for High and Vocational Schools 4to, 

Principles of Engineering Drawing 8vo, 

Lefevre, L. Architectural Pottery 4to, 

Lehner, S. Ink Manufacture Bvo, 

Lemstrom, S. Electricity in Agriculture and Horticulture Bvo, 

Letts, E. A. Fundamental Problems in Chemistry .-..Bvo, 






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l6 D. VAN NOSTRAND L,0.'S SHORl ^TLE CATALOG 

Le Van, W. B. Steam-Engine Indicator i6mo, o 75 

Lewes, V. B. Liquid and Gaseous Fuels 8vo, 3 00 

Carbonization of Coal 8vo, *5 oo 

Lewis Automatic Machine Rifle ; Operation of i6mo, *o 6a 

Licks, H. E. Recreations in Mathematics i2nio, 1 5J> 

Lieber, B. F. Lieber's Five Letter American Telegraphic Code .8vo, *i5 00 

Spanish Edition Bvo, '''15 00 

French Edition Bvo, *i5 00 

Terminal Index 8vo, *2 50 

— — Lieber's Appendix folio, *i5 00 

Handy Tables 4to, *2 50 



Bankers and Stockbrokers' Code and Merchants and Shippers' 

Blank Tables 8vo, *i5 00 

100,000,000 Combination Code 8vo, *io 00 

Livermore, V. P., and Williams, J. How to Become a Competent Motor- 
man i2mo, *i 00 

Livingstone,* R. Design and Construction of Commutators Bvo, 4 50 

Mechanical Design and Construction of Generators Bvo, 4 50 

Lloyd, S. L. Fertilizer Materials i2mo, 2 00 

Lockwood, T. D. Electricltyj Magnetism, and Electro-telegraph .. .8vo, 250 

- — Electrical Measurement and the Galvanometer i2mo, o 75 

Lodge, O. J. Elementary Mechanics i2mo, i 50 

Loewenstein, L. C, and Crissey, C. P. Centrifugal Pumps 500 

Lomax, J. W. Cotton Spinning i2mo, i 50 

Lord, R. T. Decorative and Fancy Fabrics Bvo, *3 50 

Loring, A. E. A Handbook of the Electromagnetic Telegraph. . .i6mo, o 75 

Lowy, A. Organic Type Formulas o 10 

Lubschez, B. J. Perspective i2mo, 2 00 

Lucke, C. E. Gas Engine Design 8vo, ♦$ 00 

Power Plants: Design, Efliciency, and Power Costs. 2 vols. 

(In Preparation.) 

Luckiesh, M. Color and Its Application Bvo, 350 

Light and Shade and Their Applications Bvo, 3 00 

Visual Illusions (In Preparation.) 

Lunge, G. Coal-tar and Ammonia. Three Volumes Bvo, *25 00 

Technical Gas Analysis Bvo, *4 50 

Manufacture of Sulphuric Acid and Alkali. Four Volumes. . . .8vo, 

Vol. I. Sulphuric Acid. In three parts (Reprinting.) 

Vol. I. Supplement Bvo (Reprinting.) 

Vol. n. Salt Cake, Hydrochloric Acid and Leblanc Soda. In two 

parts (In Press.) 

Vol. III. Ammonia Soda (In Press.) 

Vol. rV. Electrolytic Methods (In Press.) 

Technical Chemists' Handbook i2mo, leather, *4 00 

Technical Methods of Chemical Analysis. 

Vol. I. In two parts Bvo (New Edition in Press.) 

Vol. II. In two parts Bvo (New Edition in Press.) 

Vol. m. In two parts Bvo (New Edition in Press.) 

The set (3 vols.) complete 



D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 17 

Luquer, L. M. Minerals in Rock Sections 8vo, i 75 

MacBride, J. D. A Handbook of Practical Shipbuilding, 

i2mo, fabrikoid, 2 00 

Macewen, H. A. Food Inspection 8vo, *2 50 

Mackenzie, N. F. - Notes on Irrigation Works 8vo, *2 50 

Mackie, J. How to Make a Woolen Mill Pay 8vo, *2 00 

Maguire, Wm. R. Domestic Sanitary Drainage and Plumbing . . /.Svo, 4 00 

Malcolm, H. W. Submarine Telegraph Cable 9 00 

Malinovzsky, A. Analysis of Ceramic Materials and Methods of 

Calculation {In Press.) 

Mallet, A. Compound Engines i6mo, 

Mansfield, A. N. Electro-magnets i6mo, o 75 

Marks, E. C. R. Construction of Cranes and Lifting Machinery. i2mo, *2 75 

Manufacture of Iron and Steel Tubes i2mo, 2 50 

Mechanical Engineering Materials i2mo, *i 50 

Marks, G. C. Hydraulic Power Engineering Svo, 4 50 

Marlow, T. G. Drying Machinery and Practice .... Svo (Reprinting.) 

Marsh, C. F. Concise Treatise on Reinforced Concrete Svo, *2 50 

,• Reinforced Concrete Compression Member Diagram. Moimted on 

Cloth Boards *i . 50 

Marsh, G. F., and Dunn, W. Manual of Reinforced Concrete and Con- 
crete Block Construction i6mo, 2 00 

Marshall, W. J., and Sankey, H. R. Gas Engines Svo, 2 00 

Martin, G. Triumphs and Wonders of Modem Chemistry Svo, *3 00 

Modern Chemistry and Its Wonders Svo, *3 00 

Martin, N. Properties and Design of Reinforced Concrete Svo, i 50 

Martin, W. D. Hints to Engineers i2mo, 2 00 

Massie, W. W., and Underbill, C. R. Wireless Telegraphy and Telephony. 

i2mo, *i 00 

Mathot, R. E. Internal Combustion Engines Svo, 5 00 

Maurice, W. Electric Blasting Apparatus and Explosives Svo, *3 50 

Shot Firer's Guide Svo, *i 50 

Maxwell, F. Sulphitation in White Sugar Manufacture, i2mo, 4 00 

Maxwell, J. C. Matter and Motion i6mo, o 75 

Maxwell, W. H., and Brown, J. T. Encyclopedia of Muni ipal and Sani- 
tary Engineering 4to, *io 00 

Mayer, A. M. Lecture Notes on Physics Svo, 2 00 

McCracken, E. M., and Sampson, C. H. Course in Pattern Making. 

(In Press.) 

McCullough, E." Practical Surveying i2mo, 3 oc 

McCuUough, R. S. Mechanical Theory of Heat Svo, 3 50 

McGibbon. W. C. Indicator D'ag'-ams for Marine Engineers Svo, *3 50 

Marine Engineers* Drawing Book oblong 4to, *2 50 

McGibbon, W. C. Marine Engineers Pocketbook i2mo, *4 50 

Mcintosh, J. G. Technology of Sugar Svo, *6 00 

Industrial Alcohol Svo, *3 50 

Manufacture of Varnishes and Kindred Industries. Three Volimies. 

Svo. 

Vol. I. Oil Crushing, Refining and Boiling 7 00 

Vol. 11. Varnish Materials and Oil Varnish Making 5 00 

Vol. III. Spirit Varnishes and Materials 6 00 



iS D VAN ^OSTRAND CO.'S SHORT TITLE CA.TALOG 

McKillop. M., auc WcKillop, A. D. EfiSciency Methods i2mo, i 50 

McKnight ]. D,. and Brown, A. W. Marine Multitubular Boilers *2 50 

McMaster J B. Bridge and Tunnel Centres : i6mo, o 75 

McMe'-hen, F L. Test<» for Ores, Mineral? and Metals lamo, i 50 

McNair, F. V Handbook for Naval Officers i2mo, 4 00 

Meade, A. Modern Gap Works Practice 8vo, *8 50 

Melick, C. W. Tairv Laboratory Guide i2mo, *i 25 

"Mentor." Self -Instruction for Students in Gas Supply. i2mo. 

Elementary ,. . 2 50 

Advanced ... 2 50 

- — Self-Instructior for Students in Gas Engineering. lamo. 

Elementary .... , 2 00 

Advanced 2 00 

Merivale, J. H. Notes and Formulae for Mining Students i2mo, i 00 

Merritt, Wm. H. Field Testing for Gold and Silver i6mo, leather, 2 50 

Mertens. Tactics and Technique of River Crossings 8vo, 3 00 

Mierzinski, S. Waterproofing of Fabrics Svo, 2 50 

Miessner, B. F. Radio Dynamics i2mo, *2 00 

Miller, G. A. Determinants. i6mo. 

Miller, W. J Introduction to Historical Geology i2mo, 2 50 

Milroy, M. E. W. Home Lace-making i2mo, *i 00 

Church Lace i2mo, 2 50 

Mills, C. N. Elementary Mechanics for Engineers Svo, i 25 

Mitchell, C. A. Mineral and Aerated Waters Svo, *3 00 

Mitchell, C. A., and Prideaux, R. M. Fibres Used in Textile and Allied 

Industries Svo, 3 50 

Mitchell, C. F., and G. A. Building Construction and Drawing. i2mo. 

Elementary Course - 2 50 

Advanced Course 4 50 

Monckto'" C. C. F, Radiotelegraphy Svo, 200 

Monteverde, R. D. Vest Pocket Glossary of English-Spanish, Spanish- 
English Technical Terms .64mo, leather, i 50 

Montgoi ^ ./y, J. H. Electric Wiring Specifications i6mo, *i 00 

Moore, K. C. S New Tables for the Complete Solution of Ganguillet and 

Kutter's Formula . Svo, *6 00 

Moore, Harold. Liquid Fuel for Internal Combustion Engines. . .Svo, 5 00 
Morecroft, J. H., and Hehre, F. W. Short Course in Electrical Testing. 

Svo, 2 00 

Morgan, A. P. Wireless Telegraph Construction for Amateurs. .i2mo, i 50 

Morga^n, J. D. Principles of Electric Spark Ignition Svo, 3 50 

Morrell, R. S., and Waele, A. E. Rubber, Resins, Paints and Var- 
nishes Svo (/n Press.) 

Moses, A. J The Characters of Crystals Svo, *2 00 

and Parsons, C. L. Elements of Mineralogy Svo, 4 50 

Moss, S. A. Elements of Gas Engine Design i6mo, o 75 

The Lay-out of Corliss Valve Gears i6mo, o 75 

Mulford, A. C. Boundaries and Landmarks i2mo, i 00 

Munby, A. E. Chemistry and Physics of Building Materials. .. .Svo, 2 50 

Murphy, J G. Practical Mining i6mo, i 00 

Murray, B. M, Chemical C^agents Svo, 300 

Murray, J.~ A. Soils and Manures » , 1 1 1 1 1 . i • 1 . . . . . . ■ 8vo, 2 00 

Nasmith, J. The Student's Cotton Spinning Svo, 4 50 

Recent Cotton Mill Construction i2mo, 3 00 



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D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 19 

lYeave, G. B., and Heilbron, I. M. Identification of Organic Compounds. 

i2mo, I 50 

Neilson, R. M. Aeroplane Patents 8vo, ^2 00 

Nerz, F. Searchlights 8vo (Reprinting.) 

Newbigin, M. I., and Flett, J. S. James Geikie, the Man and the 

Geologist 8vo, 

Ifewbiging, T. Handbook for Gas Engineers and Managers 8vo, 

Newell, F. H., and Drayer, C. E. Engineering as a Career. .i2mo, cloth, 

Tficol, G. Ship Construction and Calculations Svo, *io 00 

Wipher, F. E. Theory of Magnetic Measurements i2mo, 

Nisbet, H. Grammar of Textile Design Svo, 

Tf olan, H. The Telescope i6mo, 

Tforie, J. W. Epitome of Navigation (2 Vols.) octavo, 

A Complete Set of Nautical Tables with Explanations of Their 

Use octavo. 

North, H. B. Laboratory Experiments in General Chemistry i2mo, 

O'Connor, H. The Gas Engineer's Pocketbook i2mo, leather, 

Ohm, C S., and Lockwood, T. D. Galvanic Circuit i6mo, 

Olsen, J. C. Text-book of Quantitative Chemical Analysis Svo, 

Orrasby, M. T. M. Surveying i2mo, 

Oudin, M. A. Standard Polyphase Apparatus and Systems 8vo, 

Pakes, W. C. C, and Nankivell, A. T. The Science of Hygiene . .8vo, *i 75 

Palaz, A. Industrial Photometry Svo, 5 00 

Palmer, A. R. Electrical Experiments i2mo, o 75 

Magnetic Measurements and Experiments i2mo, o 75 

Pamely, C. Colliery Manager's Handbook Svo, *io 00 

Parker, P. A. M. The Control of Water Svo, 600 

Parr, G. D. A. Electrical Engineering Measuring Instruments .... Svo, *3 50 
Parry, E. J. Chemistry of Essential Oils and Artificial Perfumes. 

Vol. I. Monographs on Essential Oils 9 00 

Vol. II. Constituents of Essential Oils, Analysis 7 00 

Foods and Drugs. Two Volumes. 

Vol. I. The Analysis of Food and Drugs Svo, 

Vol.11. The Sale of Food and Drugs Acts Svo, 

and Coste, J. H. Chemistry of Pigments Svo, 

Parry, L. Notes on Alloys Svo, 

Metalliferous Wastes Svo, 

Analysis of Ashes and Alloys Svo, 

Parry, L. A. Risk and Dangers of Various Occupations Svo, 

Parshall, H. F., and Hobart, H. M. Electric Railway Engineering . 4to, 

Parsons, J. L. Land Drainage Svo, 

Parsons, S. J. Malleable Cast Iron Svo, 

Partington, J. R. Higher Mathematics for Chemical Students, .lamo, 

■ Textbook of Thermodynamics Svo, 

The Alkali Industry Svo, 

Patchell, W. H. Electric Power in Mines Svo, 

Paleraon, G. W. L. Wiring Calculations lamo, 

Electric Mine Sie^nalling Installations lamo, 

Patterson, D. The Color Printing of Carpet Yams Svo, 

Color Matching on Textiles Svo, 

Textile Color Mixing Svo, 



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50 


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50 


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50 


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20 D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 

Paulding, C. P. Condensation of Steam in Covered and Bare Pipes. .8vo, 

Transmission of Heat through Cold-storage Insulation i2mo, 

Payne, D. W. Iron Founders* Manual 8vo, 

Peddie, R. A. Engineering and Metallurgical Books i2mo, 

Peirce, B. System of Analytic Mechanics 4to, 

Linear Associative Algebra 4to, 

Perkin, F. M., and Jaggers, E. M. Elementary Chemistry i2mo, 

Perrin, J. Atoms 8vo, 

Petit, G. White Lead and Zinc White Paints 8vo, 

Petit, R. How to Build an Aeroplane Bvo, 

Pettit, Lieut. J. S. Grdphic Processes i6mo, 

Philbrick, P. H. Beams and Girders i6mo, 

Phin, J. Seven Follies of Science i2mo, 

Pickworth, C. N. Logarithms for Beginners lamo, boards, 

The Slide Rule i2mo, 

Pilcher, R, B. The Profession of Chemistry lamo, 

Pilcher, R. B., and Butler-Jones, F. What Industry Owes to Chemical 

Science lamo, 

Plattner's Manual of Blow-pipe Analysis. Eighth Edition, revised . Bvo, 

Plympton, G. W. The Aneroid Barometer i6mo, 

— -How to Become an Engineer i6mo, 

Van Nostrand's Table Book , .^. ., i6mo, 

Pochet, M. L. Steam Injectors i6mo, 

Pocket Logarithms to Four Places i6mo, 

i6mo, leather, 

Polleyn, F. Dressings and Finishings for Textile Fabrics Svo, 

Pollock, W. Hot Bulb Oil Engines and Suitable Vessels Svo, 

Pope, F. G. Organic Chemistry i2mo, 

Pope, F. L. Modern Practice of the Electric Telegraph' Svo, 

Popplewell, W. C. Prevention of Smoke Svo, 

Strength of Materials Svo, 

Porritt, B. D. The Chemistry of Rubber i2mo. 

Porter, J. R. Helicopter Flying Machine i2mo, 

Potts, H. E. Chemistry of the Rubber Industry Svo, 

Practical Compounding of Oils, Tallows and Grease Svo, 

Pratt, A. E. Economic Metallurgy (In Press.) 

Pratt, Jas. A. Elementary Machine Shop Practice (in Press.) 

Pratt, K. Boiler Draught i2mo, 

Prelini, C. Earth and Rock Excavation Svo, 

Graphical Determination of Earth Slopes Svo, 

Tunneling. New Editior Svo, 

Dredging. A Practical Treatise Svo, 

Prescott, A. B., and Johnson, 0. C. Qualitative Chemical Analysis . . Svo, 
Prescott, A. B., and Sullivan, E. C. First Book in Qualitative Chemistry. 

Prideaux, E. B. R. Problems in Physical Chemistry Svo, 

The Theory and Use of Indicators Svo, 

Prince, G. T. Flow of Water i2mo. 

Pull, E. Modern Steam Boilers Svo, 

Pullen, W. W. F. Application of Graphic Methods to the Design of 

i2mo, 

Structures i2mo, 

Injectors: Theory, Construction and Working i2mo, 

Indicator Diagrams Svo, 

Engine Testing Svo, 



+1 


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D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 21 

Purday, H. F. P. The Diesel Engine Design 8vo, 7 50 

Putsch, A. Gas and Coal-dust Firing 8vo, *2 50 

Rafter, 6. W. Mechanics of Ventilation i6mo, 

Potable Water i6mo, 

Treatment of Septic Sewage i6mo, 

and Baker, M. N. Sewage Disposal in the United States. .. .4to, 

Raikes, H. P. Sewage Disposal Works 8vo, 

Randau, P. Enamels and Enamelling 8vo, 

Rankine, W. J. M. A Manual of Applied Mechanics 8vo, 

Civil Engineering ' 8vo. 

Machinery and Millwork 8vo, 

The Steam-engine and Other Prime Movers 8vo, 

Rankine, W. J. M., and Bamber, E. F. A Mechanical Text-book. .8vo, 

Purday, H. F. P. The Diesel Engine Design 8vo (In Press.) 

Raphael, F. C. Localization of Faults in Electric Light and Power Mains. 

8vo, 

Rasch, E. Electric Arc Phenomena 8vo, 

Rathbone, R. L. B. Simple Jewellery 8vo, 

Rausenberger, F. The Theory of the Recoil Guns 8vo, 

Rautenstrauch, W. Notes on the Elements of Machine Design. 8 vo, boards, *i 50 
Rautenstrauch, W., and Williams, J. T. Machine Drafting and Empirical 
Design. 

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Part II. Empu-ical Design (In Preparation.) 

Raymond, E. B. Alternating Current Engineering i2mo, 

Rayner, H. Silk Throwing and Waste Silk Spinning Svo, 

Recipes for the Color, Paint, Varnish, Oil, Soap and Drysaltery Trades, 

Svo, 

Recipes for Flint Glass Making i2mo, 

Redfern, J. B., and Savin, J. Bells, Telephones i6mo, 

Redgrove, H. S. Experimental Mensuration i2mo. 

Reed, S. Turbines Applied to Marine Propulsion *5 00 

Reed's Engineers' Handbook 8vo, 

Key to the Nineteenth Edition of Reed's Engineers' Handbook. .8vo, 

Useful Hints to Sea -going Engineers i2mo, 

Reid, E. E. Introduction to Research in Organic Chemistry. (In Press.) 
Reinhardt, C. W. Lettering for Draftsmen, Engineers, and Students. 

oblong 4to, boards, i 25 
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oblong, 4to, boards, *i 00 

Reiser, F. Hardening and Tempering of Steel i2mo, 2 50 

Reiser, N. Faults in the Manufacture of Woolen Goods 8vo, 2 50 

Spinning and Weaving Calculations .8vo, *5 00 

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Reuleaux, F. The Constructor 4to, 4 00 

Rey, Jean. The Range of Electric Searchlight Projectors Svo, 

(Reprinting.) 

Reynolds, 0., and Idell, F. E. Triple Expansion Engines i6mo, o 75 

Rhead, G. F. Simple Structural Woodwork i2mo, *i 25 

Rhead, G. W. British Pottery Marks 8vo, 

Rhodes, H. J. Art of Lithography 8vo, 

Rice, J. M and Johnson, W. W. A New Method of Obtaining the Differ- 
ential of Functions i2mo, 



*2 


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5 00 



o 50 



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Richards, W. A. Forging of Iron and Steel i2mo, 

Richards, W. A., and North, H. B. Manual of Cement Testing i2mo, 

Richardson, J. The Modern Steam Engine 8vo, 

Richardson, S. S. Magnetism and Electricity i2mo, 

Rideal, E. K. Industrial Electrometallurgy 8vo, 

The Rare Earths and Metals 8vo (In Press.) 

Ozone 8vo, 

Rideal, S. Glue and Glue Testing 8vo, 

The Carbohydrates 8vo {In Press.) 

Riesenberg, F. The Men on Deck i2mo, 

Standard Seamanship for the Merchant Marine. i2mo (In Press.) 

Rimmer, E. J. Boiler Explosions, Collapses and Mishaps 8vo, 

Rings, F. Reinforced Concrete in Theory and Practice lamo, 

Reinforced Concrete Bridges 4to, 

Ripper, W. Course of Instruction in Machine Drawing folio, 

Roberts, F. C. Figure of the Earth i6mo, 

Roberts, J., Jr. Laboratory Work in Electrical Engineering 8vo, 

Robertson, J. B. The Chemistry of Coal lamo, 

Robertson, L. S. Water-tube Boilers 8vo, 

Robinson, J. B. Architectural Composition 8vo, 

Robinson, S. W. Practical Treatise on the Teeth of Wheels. .i6mo, 

Railroad Economics i6mo, 

Wrought Iron Bridge Members i6mo, 

Robson, J. H. Machine Drawing and Sketching 8vo, 

Roebling, J. A. Long and Short Span Railway Bridges folio. 

Rogers, A. A Laboratory Guide of Industrial Chemistry 8vo, 

.« Elements of Industrial Chemistry lamo, 

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Rogers, F. Magnetism of Iron Vessels i6mo, o 75 

Rohland, P. Colloidal and Crystalloidal State of Matter i2mo, 

(Reprinting.) 

RoUinson, C. Alphabets Oblong, i2mo, 

Rose, J. The Pattern-makers' Assistant 8vo, 

— — Key to Engines and Engine-running i2mo, 

Rose, T. K. The Precious Metals 8vo, 

Rosenhain, W. Glass Manufacture 8vo, 

Physical Metallurgy, An Introduction to 8vo, 

Roth, W. A. Physical Chemistry 8vo, 

Rowan, F. J. Practical Physics of the Modern Steam-boiler 8vo, 

and Idell, F. E. Boiler Incrustation and Corrosion i6mo, 

Roxburgh, W. General Foundry Practice 8vo, 

Ruhmer, E. Wireless Telephony 8vo, 

Russell, A. Theory of Electric Cables and Networks Svo, 

Rutley, F. Elements of Mineralogy i2mo, 

Rust, A. Practical Tables for Navigators and Aviators 8vo, 

Sandeman, E. A. Notes on the Manufacture of Earthenware. . .i2mo, 

Sanford, P. G. Nitro-explosives 8vo, 

Saunders, C. H. Handbook of Practical Mechanics i6mo, 

leather, 

Sayers, H. M. Brakes for Tram Cars 8vo, 

Schaef er, C. T. Motor Truck Design Svo, 

Scheele, C. W. Chemical Essays Svo, 



2 


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D. VAN NOSTRAND CO.'S SHORT TITLE CATAlLOG 23 

Scheithauer, W. Shale Oils and Tars 8vo, 

Scherer, ,R. Casein 8vo, 

Schidrowitz, P. Rubber, Its Production and Industrial Uses 8vo, 

Sciiindler, K. Iron and Steel Construction Works lamo, 

Schmall, C. N. First Course in Analytic Geometry, Plane and Solid. 

i2mo, half leather, 

and Shack, S. M. Elements of Plane Geometry i2mo, 

Schmeer, L. Flow of Water Svo, 

Schwarz, E. H. L. Causal Geology Svo, 

Schweizer, V. Distillatiens of Resins Svo, 

Scott, A. H. Reinforced Concrete in Practice lamo, 

Scott, W. W. Qualitative Analysis. A Laboratory Manual. New 

Edition ., 

Standard Methods of Chemical Analysis Svo, 

Scribner, J. M. Engineers' and Mechanics* Companion. .i6mo, leather, 
Scudder, H. Electrical Conductivity and Ionization Constants of 

Organic Compounds Svo, 

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Searle, A. B. Modern Brickmaking Svo, 

. Cement, Concrete and Bricks Svo, 

Searle, G. M. "Sumners' Method." Condensed and Improved. 

i6mo, 

Seaton, A. E. Manual of Marine Engineering. Svo, 

Seaton, A. E., and Rounthwaite, H. M. Pocket-book of Marine Engi- 
neering , i6mo, leather, 

Seeligmann, T., Torrilhon, G. L., and Falconnet, H. India Rubber and 

Gutta Percha Svo, 

Seidell, A. Solubilities of Inorganic and Organic Substances .... Svo, 

Sellew, W. H. Steel Rails 4to, = 

Railway Maintenance Engineering lamo, 

Senter, G. Outlines of Physical Chemistry i2mo, 

Text-book of Inorganic Chemistry > lamo, 

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Sever, G. F., and Townsend, F. Laboratory and Factory Tests in Elec- 
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Lessons in Telegraphy lamo, 

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Chemistry of the Materials of Engineering 12 mo, 

Alloys (Non-Ferrous) Svo, 

Sexton, A. H., and Primrose, J. S. G. The Metallurgy of Iron and Steel. 

Svo, 

The Common Metals (Non-Ferrous) Svo, 

Seymour, A. Modern Printing Inks Svo, 

Shaw, Henry S. H. Mechanical Integrators i6mo, 

Shaw, S. History of the Staffordshire Potteries .Svo, 

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Shaw, T. R. Driving of Machine Tools lamo, 

Precision Grinding Machines lamo, 

Shaw, W. N. Forecasting Weather Svo (Reprinting.) 

Sheldon, S., and Hausmann, E. Dynamo Electric Machinery, A.C. 

and D.C Svo (In Press.) 

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24 D. VAN NOSTRAND CO/S SHORT TITLE CATALOG 

Sherriff, F. F. Oil Merchants' Manual and Oil Trade Ready Reckoner, 

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Shunk, W. F. The Field Engineer lamo, fabrikoid, 3 00 

Silverman, A., and Harvey, A. W. Laboratory Directions and Study 

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Simmons, W. H., and Appleton, H. A. Handbook of Soap Manufacture, 

8/0, *4 CO 

Simmons, W. H., and Mitchell, C. A. Edible Fats and Oils 8vo, 

Simpson, G. The Naval Constructor i2mo, fabrikoid, 

Simpson, W. Foundations. . .' 8vo. (In Press.) 

Sinclair, A. Development of the Locomotive Engine. . . 8vo, half leather, 

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Sindall, R. W., and Bacon, W. N. The Testing of Wood Pulp 8vo, 

Wood and Cellulose 8vo (In Press.) 

Sloane, T. O'C. Elementary Electrical Calculations. lamo, 

Smallwood, J. C. Mechanical Laboratory Methods. .. .lamo, fabrikoid, 
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Smith, C. A. M., and Warren, A. G. New Steam Tables Svo, 

Smith, C. F. Practical Alternating Currents and Testing Svo, 

Practical Testing of Dynamos and Motors Svo, 

Smith, F. E. Handbook of General Instruction for Mechanics . . . i2mo. 
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Manufacture and Properties i2mo, 

Smith, H. G. Minerals and the Microscope i2mo. 

Smith, J. C. Manufacture of Paint Svo, 

Smith, R. H. Principles of Machine Work i2mo, 

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Snow, W. G., and Nolan, T. Ventilation of Buildings i6mo, o 75 

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Solomon, M. Electric Lamps Svo, 2 00 

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Sothern, J. W. The Marine Steam Turbine 8vo, 

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Southcombe, J. E. Chemistry of the Oil Industries 8vo, 

Soxhlet, D. H. Dyeing and Staining Marble Svo, 

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Surve3nng i6mo, 

Spencer, A. S. Design of Steel -Framed Sheds Svo, 

Spiegel, L. Chemical Constitution and Physiological Action lamo. 



*3 

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D. VAN NOSTRAND CO.'S SHORT TITLE CATALOG 25 

Sprague, E. H. Hydraulics lamo, 

Elements of Graphic Statics 8vo, 

Stability of Masonry lamo, 

Elementary Mathematics for Engineers. lamo, 

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Strength of Structural Elements i2mo, 

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Stahl, A. W. Transmission of Power i6mo, 

Stahl, A. W., and Woods, A. T. Elementary Mechanism lamo, 

Standage, H. C Leatherworkers' Manual 8vo, 

Sealing Waxes, Wafers, and Other Adhesives 8vo, 

Agglutinants of All Kinds for All Purposes i2mo, 

Stanley, H. Practical Applied Physics {In Press.) 

Stansbie, J. H. Iron and Steel Svo, 

Steadman, F. M. Unit Photography i2mo, 

Stecher, G. E. Cork. Its Origin and Industrial Uses i2mo, 

Steinheil, A., and Voit, E. Applied Optics* Vols. I. and II. Svo, 

Each, 

Two Volumes Set, 

Steinman, D. B. Suspension Bridges and Cantilevers. (Science Series 

No. 127.) 075 



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Stevens, A. B. Arithmetric of Pharmacy i2mo, i 50 

Stevens, E. J. Field Telephones and Telegraphs i 20 

Stevens, H. P. Paper Mill Chemist i6mo, 4 00 

Stevens, J. S. Theory of Measurements i2mo, 

Stevenson, J. L. Blast-Furnace Calculations i2mo, leather, 

Stewart, G. Modern Steam Traps i2mo. 

Stiles, A. Tables for Field Engineers i2mo, 

Stodola, A. Steam Turbines Svo, 

Stone, E. W. Elements of Radiotelegraphy x2mo, fabrikoid. 

Stone, H. The Timbers of Commerce 8vo, 

Stopes, M. The Study of Plant Life Svo, 

Sudborough, J. J., and James, T. C. Practical Organic Chemistry. i2mo, 

Suf fling, E. R. Treatise on the Art of Glass Painting Svo, 

Sullivan, T. V., and Underwood, N. Testing and Valuation of Build- 
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Svenson, C. L. Handbook on Piping Svo, 

Essentials of Drafting Svo, 

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Swan, K. Patents, Designs and Trade Marks Svo, 

Swinbume,.J.,Wordingham,C.H., and Martin, T.C. Electric Currents. 

i6mo, 

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Tailf er, L. Bleaching Linen and Cotton Yarn and Fabrics Svo, 

Tate, J. S. Surcharged and Different Forms of Retaining-walls. .i6mo, 

Taylor, F. N. Small Water Supplies i2mo, 

Masonry in Civil Engineering Svo, 

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Tayloi, W. T Calculation of Electrical Conductors 4to, 

Electric Power Conductors and Cables Svo {In Press.) 

Calculation of Electric Conductors 4to {In Press.) 

Templeton, W. Practical Mechanic's Workshop Companion. 

i2mo, morocco, 

Tenney, E. H Test Methods for Steam Power Plants i2mo, 

Terry.. H. L. India Rubber and its Manufacture Svo, 



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50 



*3 50 


1 


50 


2 


50 


3 


oo 


10 


oo 


15 


00 





75 


I 
*3 


50 

00 


*2 


25 


*3 


00 


*3 


50 



26 D. VAN NOSTRAND CO/S SHORT TITLE CATALOG 

Thayer, H. R. Structural Design. 8vo. 

Vol. I. Elements of Structural Design.... 350 

Vol. n. Design of Simple Structures 450 

Vol. III. Design of Advanced Structures {In Preparation,) 

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Thiess, J. B., and Joy, G. A. Toll Telephone Practice 8vo, 

Thom, C, and Jones, W. H. Telegraphic Connections.. . .oblong, i2mo, 

Thomas, C. W. Paper-makers' Handbook (In Press.) 

Thomas, J. B. Strength of Ships 8vo, 

Thomas, Robt. G. Applied Calculus lamo, 

Thompson, A. B. Oil Fields of Russia 4to, 

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Thompson, S. P. Dynamo Electric Machines i6mo, 

Thompson, W. P. Handbook of Patent Law of All Countries i6mo, 

Thomson, G. Modem Sanitary Engineering i2mo, 

Thomson, G. S. Milk and Cream Testing. i2mo, 

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Tinney, W. H. Gold-mining Machinery Svo, 

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Townsend, J. S. Ionization of Gases by Collision Svo, 

Transactions of the American Institute of Chemical Engineers, Svo. 

Vol. I. to XL, 1908-1918 Svo, each. 

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2 


50 


4 


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♦3 


00 


2 


50 


4 


50 


2 


50 


*2 


00 


12 


00 


*0 


75 


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25 


6 


00 





75 


2 


00 


3 


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Watson, E. P. Small Engines and Boilers 121110, i 25 

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Wimperis, H. E. Internal Combustion Engine. . , Svo, 3 50 

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Wright, J. Testing, Fault Finding, etc., for Wiremen i6mo, ^ 50 

Wright, T. W. Elements 0* Mechanics 8vo, *3 00 

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